i had also freezes in the past.
it turned out to be a defective cpu!
i couldnt believe it, coz it didn't crash all the time, just with 3d-games.
i inserted new cpu, an all problems went away.
try to run the prime95 program (the torture-test and the benchmark) (http://www.mersenne.org
if it fails, crashes or is reporting a defective hardware, then it is most likely you cpu or ram or mainboard.
in my case it was the cpu.
i hope this helps.