Flickering/Fuzzing/Dithering on Blacks/Greys via HDMI. PC Monitor

If it's a pixel-sized checkerboard pattern, then it's the frame-rate control (FRC) in the display panel at work dithering the picture to simulate more intermediate shades than the panel natively supports. It's a brute force method of improving gradation where greater precision (more bits in the DAC for smaller voltage changes) would no longer help align the liquid crystal as desired.

It's particularly noticeable on TN panels common in PC monitors. Other panel types use dithering as well, though the way they align the liquid crystal may allow for higher native precision and make dithering less noticeable.

Really common artifact when playing games that render at 720 or below that then have to be upscaled by the 360. When you are playing is the 360 set to output 1080? It's possible that you could set the image output to 720 and let your monitor handle the scaling instead of the 360. For whatever reason the 360 has been known for black gradient artifacts. On the old forums there was a thread about what could be causing it. Seems like a hardware issue as it shows up in multiple games with different engines.

Thanks for your replys guys. So this is an issue with FRC? Makes sense, it just seems to happen much worse on gradients.

But it also happens in Windows, not just an Xbox issue it seems, it may just be a MONITOR issue.

Has anyone else noticed this?

Have you tried using different HDMI cables? I just bought a cable and when I connected it to my TV and some black parts started flickering as well.

I've tried a different HDMI cable...

No one else notices this when using a PC Monitor? :/

I know you said your monitor is full HD; which I'm assuming it has the capability of doing 1080p.  Now have you tried using different resolutions?