Unknown Problem: lockups

i built a computer for my friend almost everything went right. i found out that the 6600gt was supposed to be 50-85C so thats one problem down.

the next problem we came across is that sometimes the computer will just freeze. similar to those crappy mac computers i used in school...
anyway first it was when he was flying in bf2, happened after afew minutes.
so i figured it was a card overheating, but the temps were supposedly fine. i underclocked and the same problem happened.
then it happened when he was just infantry, if any of you know bf2 thats probly 20% less stress, maybe more.
then again this morning just browsing the internet.
if anyone knows of programs that test your hardware that would be great.

it could be faulty RAM it could be overheating video card, it could be... alot of things.

thanks
Brian