Slowdown with nvidia cards after 5 minutes of play

General Nvidia slowdown

I've recently started experiencing a problem with nvidia graphics cards where games will run well for 5 minutes or so and then lose between 5 and 20 fps depending on the onscreen action. Eventually, they seem to drop even further. Alt tabbing out of the program and back in appears to fix things for a VERY short amount of time, but it always goes back to where it was before.

I heard this was a problem with the 8800 gts series around a year ago, but it seems like I'm still having the same problem today. I tried rolling back to the stock drivers to no avail, and have worked with several different driver versions, including the most recent ones. Nothing seemed to work.

I upgraded to a GTX 260, thinking this might improve things, but other than raising FPS across the board, it still has problems dropping performance after 5-10 minutes of play. This is all off a fresh reformat and install of windows, too. Here are my specs

Windows XP 32 Bit SP3
Intel Core 2 Duo E6850 @ stock clock of 3.0 ghz
BFG GTX 260 896mb (factory overclocked)
650 watt PSU
Sound Blaster Audigy Gamer (from way back in 2002)
2 GB DDR2 SDRAM
Gigabyte s series GA-P35-DS3L motherboard

My system is really stable other than this strange problem. I don't think it's my power supply because 650 watts should have been more than enough for the 8800 gts and I still had the same problem. It's really frustrating because a fairly high end system can run a game from 2 years ago like The Wither without dropping to about 20fps after several minutes of play. Any suggestions?