This problem is very new, less than a week old (no major internal changes since leading to it or anything). When I'm casually using the computer (Word, design programs, internet, even Minecraft [simple game]) it runs fine.
However as soon as I start up a newer game, ranging from Evil Genius (2004) to the new Medal of Honor Beta (2010), I can play for about 10-60 minutes (it varies) at which point my sound crackles or starts repeating and my monitor loses signal. The reason I know the HD is active is that I'm typically on Ventrilo and my friends say I'm still logged in when the problem occurs. Occasionally the sound doesn't crap out and it's just the video.
At this point the reset button doesn't work either, I have to do a hard shut down to get a successful reboot.
Friends have suggested bad PSU (but I've run Burnit twice and it finished both times) and bad MoBo, but I'm at a loss for what this is. The only way to replicate it is to play a game and just wait, and there's no way I know of to get an error report (nothing in the Windows event log either, I don't think).