My fault finding would go in the order:
Are the correct/latest drivers installed - I would go as far as opening the case and reading the writing on the video card to make sure it is what you think it is, chipset etc.
Google the chipset and see if anyone else has reported problems with the game and your video card/chipset.
Is the video card overheating - games tend to run video GPU's to their extreme. Try running it with case open and a desk fan blowing into the case.
Is the game corrupt - will it run on another computer. Hackers have been know to put malicious code in ripped off games.
Is it a RAM fault on the main system board rather than the video card? Have you done a memory test.