there might b two reasons :
1) is your processore overclocked? do a prime95 test for a few hours n check if the system remains stable. if it is an issue, then u might need 2 tweak the voltages &/or timings.
2) try downloading the latest patches of the game.
Also, wat OS do you have?
Not overclocked. Everything in my system is stock. I'm using windows 7 64x. I think I have the latest patches because it wouldn't let me connect to multiplayer if I didn't have it.