[andrewas]

Three things come to mind.

Overheating. If its a fairly standard design open up the case and see if the temps drop and more importantly if the problem clears up. A more permanent fix would involve clearing up the wiring and removing dust, and then having a look at airflow and heatsink quality and all sorts of fun things. But any shop-bought system should run at stock speed with stock cooling if you keep it in decent condition. You can use the Motherboard Monitor 5 program to see you temperatues, but if you have software specificaly for your system, that will be easier to use.

Dodgy PSU. MBM5 can show the voltages, look at the 12 an 5 volt rails, typicaly the 5 volt will drop down to about 4.4-4.2 and then crash. Dosent tend to cause freezes though.

Dodgy RAM. Sisoft Sandra, run the memory burnin test for a while. And I do not mean one cycle. Or, more simply, take out all but one stick and see if you have problems, and switch around till you identify the dead stick.

Or it could be some dodgy drivers, some malware (run adaware, spybotSD, trend housecall), some game specific problem.