You mean the random artifacts showing up?
There are several causes of this problem.
Clean your PC out first, following the steps
here.Then, if that doesn't help, reinstall your drivers.
1. Download the new drivers, but don't install them.
2. Uninstall the old drivers.
3. Reboot.
4. Install new drivers.
5. Reboot.
See if that helps.
If neither of those help, try reinstalling the game.
If that doesn't help, post back and we'll try and help you further.