when GPU stressed PC crashes and reboots

I'm not sure from your description if your system simply re-boots when it "crashes", but if you are seeing a Blue Screen of Death (BSOD) then I'd suggest analyzing the dump files from your BSODs as a first troubleshooting. See my 07-Jan-2022 post in About the BSOD on Windows Vista for information about free utilities like Nirsoft's BlueScreenView and Resplendence's Who Crashed that you can use to interpret and summarize the Bug Check (BC) codes in your recent Windows mini-dump files. I've had a few BSODs on my Vista SP2 machine in past years that were caused by an outdated NVIDIA graphics driver (see the BlueScreenView image below) and if you're lucky one of these free utilities might point you to the driver or module causing your crashes. You didn't explain why you "theorise it could be a problem with the south bridge" but if this means you think there's a problem with your AMD FX-4100 CPU (launched Oct 2011) then you might want to ask for help in BleepingComputer's free Windows Crashes and Blue Screen of Death (BSOD) Help and Support board that I mentioned in that post and have someone there perform a proper analysis of your full dump files. The volunteers that monitor that BleepingComputer board might even be able to diagnose your problem if your crashes don't involve a BSOD.

If you require further assistance let us know your computer make/model, antivirus and default browser. If you have a Vista SP2 OS is it 32-bit or 64-bit, and are you patched to end-of-support on 11-Apr-2017 for this OS? Could you give an example of what GPU intensive program(s) you are using when your system crashes (e.g., are you gaming, watching videos or performing some other graphics-intensive task, and are you using your browser for these tasks)?

If your crashes occur while you are using your browser have you tested with hardware (graphics) acceleration disabled in your browser?

Is your GeForce GTX Titan Black graphics card (launched Feb 2014) the original discrete graphics card that shipped from the factory with your machine, and what is your current NVIDIA graphics driver version? I can't find the GeForce GTX Titan Black listed on NVIDIA's driver search page at https://www.nvidia.com/en-us/geforce/drivers/, and the closest match I could find was for a GTX 780 Ti (similar to the GTX Titan Black but with 3 GB RAM); the most recent WHQL drivers listed for the GTX 780 Ti are v365.19 (rel. 13-May-2016) for 32-bit Vista and v309.08 (rel. 24-Feb-2015) for 64-bit Vista.

You might also want to post about your problem in the GeForce Graphics Card board at https://www.nvidia.com/en-us/geforce/forums/geforce-graphics-cards/5/. I searched that forum for "GTX Titan Black" and found a few threads like pandahbee's 2019 thread No Signal From GPU When It’s Still Receiving Power that might have some helpful troubleshooting hints.

Just based on your brief description it sounds like it might be a thermal issue. Even if your CPU/GPU temps aren't reaching a critical temp that serious enough to shut down your computer you could still have a weak solder point or loose connector on your motherboard that's expanding and causing a problems when your system heats up.

I'm not a computer hardware expert, but if you want to publish a Speccy snapshot of your system I'll take a quick look at your system configuration to see if any obvious problem jumps out at me. To post a Speccy snapshot:

1. Install Piriform's free Speccy utility (available at https://www.ccleaner.com/speccy).

2. Launch the program and allow it to collect basic information about your system software and hardware.

3. Go to File | Publish Snapshot and copy and paste the URL this generates (e.g., http: // speccy.piriform.com/results/xxx... as shown in the image below) in your next reply.

If you don't want to install Speccy there is a portable version (currently spsetup132.zip) available for download <here> that can be saved and unzipped on a removable USB thumb drive - just double-click the appropriate executable to launch the program (Speccy.exe for a 32-bit OS; Speccy64.exe for a 64-bit OSs). A Speccy diagnostic isn't as thorough as the Farbar Recovery Scan Tool (FRST) diagnostic described in the Malwarebytes support article Run Farbar Recovery Scan Tool to Gather Logs but your Speccy snapshot will give us some basic information about your current hardware and software.

If you run a FRST diagnostic be sure to check through the Event Log and Code Integrity errors listed at the bottom of the Addition.txt log and see if you can match any of those errors to the date and time of your shutdowns.
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32-bit Vista Home Premium SP2 * Firefox ESR v52.9.0 * Malwarebytes Premium v3.5.1.2522-1.0.365
HP Pavilion dv6835ca, Intel Core2Duo T5550 @ 1.83 GHz, 3 GB RAM, 256 GB WD SATA HDD, NVIDIA GeForce 8400M GS
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64-bit Win 10 Pro v21H2 build 19044.1466 * Firefox v96.0.1 * Microsoft Defender v.4.18.2111.5-1.1.18800.4 * Malwarebytes Premium v4.5.0.152-1.0.1538
Dell Inspiron 15 5584, Intel i5-8265U CPU, 8 GB RAM, 256 GB Toshiba KBG40ZNS256G NVMe SSD, Intel UHD Graphics 620

Thanks for the link to your Speccy snapshot. I'll review it later and get back to you if I see anything unusual, but I did notice that you have a Win 7 SP1 OS (note that you posted your question in the Windows Vista board of this forum). Is your OS patched to end of support on 14-Jan-2020, and do you know if the KB4474419 update (rel. 23-Sep-2019) that adds SHA-2 code-signing support has been installed? If you aren't sure go to Control Panel | Programs and Features | View Installed Updates and search for KB4474419 (i.e., don't search for a partial string like "4474419").

One other thought, since you have a Win 7 SP1 OS and AMD FX-4100 CPU and are not using the original graphics card that shipped from the factory. What is the make/model of the original graphics card that came with your computer (you still haven't answered my question about your computer make/model), and does your computer model support hybrid (switchable) graphics where the system can automatically switch between the on-board GPU for less demanding tasks and the high-end discrete graphics card for graphics-intensive tasks (e.g., gaming) to optimize power efficiency? See the AMD support article How to Configure Laptop Switchable Graphics for more information about switchable graphics.

I recall there were several Win 7 SP1 users who experienced BSODs related to an out-of-date AMD Radeon graphics card driver (igdpmd64.sys) on computers configured with hybrid (switchable) graphics after Windows Update installed KB2670838 - see sven74's Dec 2013 thread NIS 2014 BSOD After Junk Update from Windows in the Norton forum for one example. The Windows support article Platform Update for Windows 7 SP1 and Windows Server 2008 R2 SP1 included a list of Dell, HP, Lenovo and Sony computer models that shipped with AMD Radeon graphics cards that supported hybrid (switchable) graphics and were affected by this problem.

I understand that you are not seeing BSODs when your PC re-boots, but if your computer model / motherboard supports hybrid (switchable) graphics to optimize power efficiency and you are not using the original graphics card that shipped from the factory that might explain why your computer is re-booting when your system attempts to switch to your high-end discrete graphics card during a graphics-intensive task.
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64-bit Win 10 Pro v21H2 build 19044.1466 * Firefox v96.0.1 * Microsoft Defender v.4.18.2111.5-1.1.18800.4 * Malwarebytes Premium v4.5.0.152-1.0.1538
Dell Inspiron 15 5584, Intel i5-8265U CPU, 8 GB RAM, 256 GB Toshiba KBG40ZNS256G NVMe SSD, Intel UHD Graphics 620

According to your Speccy snapshot you haven't installed any Win 7 SP1 updates since 2016, which implies that the KB4474419 update (rel. 23-Sep-2019) that provides SHA-2 code signing support is not installed on your system. When I searched for the latest 64-bit Win 7-compatible driver for your NVIDIA GTX Titan Black graphics card at https://www.nvidia.com/Download/index.aspx?lang=en-us it recommended the v472.12 driver (rel. 20-Sep-2021), but the release notes <here> state ...

"Requirement for Obtaining NVIDIA Drivers for Windows 7

Microsoft now provides only SHA-2 signed drivers. If your Windows 7 system is NOT equipped to detect SHA-2, you need to install the SHA-2 update support patches. For details as well as access to SHA-2 update support patches, see the Microsoft KBA "2019 SHA-2 Code Signing Support requirement for Windows and WSUS"

... so I'm not sure how you managed to install the latest v30.0.14.7212 (a.k.a. v472.12) driver without SHA-2 code-signing support.

You still haven't answered many of my basic questions about your system (e.g., your current antivirus and default browser) but many third-party programs won't update these days on Win 7 SP1 computers unless SHA-2 code signing support is installed, so getting your OS patched to end of support on 14-Jan-2020 should be a priority. See my 05-Dec-2021 reply in the Norton forum thread Windows 7 Install Failure (where I post as user lmacri) that has links to instructions for patching a clean reinstall of Win 7 back to end of support on 14-Jan-2020, as well as abbreviated instructions for users who have most of their Win 7 SP1 updates and only need to install KB4474419 to add SHA-2 code signing support. Also note that Windows Update will not install any updates released for Win 7 SP1 on or after July 2019 until KB4474419 is installed.

Regarding the Windows Error Recovery prompt you sometimes see at boot-up, once your OS is patched to end of support I'd recommend running a thorough Check Disk (chkdsk /r) of your C: drive from an elevated command prompt with Administrator rights to see if your OS can recovery any system files that might have been written to a bad sector [if you answer "Y" (yes) to scheduling the scan note that you will be prompted to restart your computer; once started, the scan might take a few hours to run to completion depending on the size of your hard drive]. Then run a System File Checker scan (sfc /scannow) to see if it can find and repair any missing or corrupted system files. If System File Checker does not find any issues with your system files it will report that "Windows Resource Protection did not find any integrity violations." From my old Vista SP2 machine:

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Just a side note, but do your Steam games or other software require Java, and if so is there any reason why you still have the Java Runtime Environment (RE) plugin v1.8.0.101 (a.k.a. Version 8 Update 101, rel. 19-Jul-2016) installed in your browser when Version 8 Update 311 (rel. 19-Oct-2021) is available for download? I removed the Java RE plugin from my browsers several years ago because of the danger of being infected by Java-based malware, but if you absolutely need the Java RE on your system I'd suggest you update your plugin and only enable it in your browser when required.
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64-bit Win 10 Pro v21H2 build 19044.1466 * Firefox v96.0.1 * Microsoft Defender v.4.18.2111.5-1.1.18800.4 * Malwarebytes Premium v4.5.0.152-1.0.1538
Dell Inspiron 15 5584, Intel i5-8265U CPU, 8 GB RAM, 256 GB Toshiba KBG40ZNS256G NVMe SSD, Intel UHD Graphics 620