Do you have any overclocked components? This sounds like 1) A driver problem, or 2) A heat related problem.
My money is on a driver problem, though. If it was heat related, most likely your computer would just radomly restart during gameplay.
Try downloading an
earlier driver for your card
Also, to see if it is heat related, use
MBM 5 to check your temperatures when the game freezes, or shortly after restart, but I believe MBM 5 will alert you if your temps get too high, automatically prooving it is a heat problem.
If it is a heat problem (or if it isn't, you should do this anyways), carefully vacuum out your case and fans, especially the CPU fan, because the CPU overheating would be the main culprit in a heat related freeze.