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I can't say that I know what would cause the screen flicker.
On the subject of task managers: Most of the applications on the phone don't run in the background. They may appear to never have closed because they save their state and can restore their state when you navigate back to them. If it's not in use, it's not
in memory. The exception to this would be certain system functionality that either always runs or that will run continuously when in use, such as the media player (it can still play while you are doing other things) or the process that checks e-mail.
I've had a similar issue. After the phone has been up for a while (24h or so) when I return to the start menu after being in an app (Flowerz once and Pictures the second time) the black background was replaced by a flickering patchwork image of the app
I'd just been in. Didn't seem to affect phones performace, but didn't go away until I'd taken the battery out and reset.
This is happening to my device too. I have a HTC HD7. Pretty scary, I was considering taking the device back to the store I bought it from, but if it's happening to lots of people then I guess it's not just my device, and so is presumably a software fail.
Either way it's very bad and I hope it gets fixed soon.