Okay, I've had this problem for awhile now. My 8600 GT (which I recently re-installed drivers for) seems to "glitch" randomly while I'm playing games. It doesn't matter if I just turn the computer on for the day, or if it's been on for hours. When I'm doing anything like playing a game or even surfing a web page with animations on it, the screen will all of a sudden freeze and get all pixely, and a low buzzing sound will play. It's even done it while using the visualizer in iTunes. Here are some pictures (taken with a digital camera) for more detail. Sorry for the size.
on a website with animated gifs
assassin's creed loading screen (notice the random dots that aren't supposed to be there)
I have made sure the drivers are up to date, and also that the card is in the slot properly and tightly. If anyone knows how to fix this problem, please let me know. Thanks