If I start a game, let's say COD4 for example, I can play that game perfectly for maybe 1-2 minutes MAX before the framerate suddenly drops through the floor and it's almost unbearable. If I close the game, the frame rate for everything is very poor - scrolling down pages in Firefox, for example. Very choppy. The only fix is a restart. After restarting, everything runs perfectly all day without frame rate issues until I launch a game, then after 30-60 seconds or so of gameplay with 100+ FPS, it suddenly crashes below 20. I uninstalled the older drivers I had through Control Panel, removed the drivers through device manager, and restarted in safe mode to use driver cleaner. Restarted and installed the most recent drivers+CCC. It's a BRAND NEW video card. No overclocking, so the stock cooling should be more than adequate. I have excellent cooling and circulation in the case itself. On my older x1800XT I had 0 problems. Does anyone have any ideas?

Pentium D 950
4gb DDR2 800
Abit AW8
650w PSU (38a on the 12+)
Vista x64
Sapphire 4850