I haven't really used my computer for gaming for a while until recently with Max Payne 3 and Cryengine 3, so I'm not sure what changes could cause this. I noticed my performance in MP3 was ~70fps one day and then 16fps the next day on the same settings and the Cryengine SDK was getting around 10 frames per second regardless of what quality settings I use. Rebooting always fixes the problem.
What can cause this? My physical memory is not above 50% usage and my CPU is pretty much idle (according to the windows task manager)
EDIT: OS is probably important, Win 7 x64
Q6600@3.0, 4 gigs ddr2, gtx460 768 not that it matters.
Try putting ice cubes on top of your video card. It worked on my old pc. Also, takeout your case, and buy an annoyingly loud fan and put it right next to your naked pc. I would put ice in it first though. If all else fails, find a 16mb video card and run crysis 2 on that. You need ice from the artic to cool that beast.