7900 gs freezing

I have a BFG 7900 gs, and my computer randomly freezes when i play video games. It has frozen on these games: counter-strike source, need for speed most wanted, and cnc 3: tiberium wars demo.
On cs:s, my screen freezes for like a minute after a few hours of playing, then runs normally.
On nfsmw, it randomly freezes for a minute, then runs again.
For the cnc3 demo, my computer completely freezes after about 30 minutes of playing, and i have to restart the entire computer.
Interestingly, my computer does not freeze on less gpu-intensive games, such as warcraft 3. I'm guessing that the freezing is caused by my gpu overheating, but i would like some professional advice before dropping 40 bucks on an aftermarket cooler. thanks.

Specs:
Core 2 Duo E6300
2 gig ddr2 ram
Windows XP Media Center Edition
Nvidia BFG 7900 gs 256mb