What are your other system specs? I mean it could be your video, it could be your hard disk, it could be your processor. Games are really video hungry, so your video card could be the culprit.

It could be bad ram. To test it use a ram tester:

http://www.memtest86.com/

Also do a defrag, reinstall the game. Patch the game.

There are other things to try later on but these would be the first that I would attempt.