What ATi card are you using? I think this is a problem possibly with the card overheating, even if you are running it at stock speeds, because artifacts to my knowledge are usually caused by cards that are oc'ed to far and either overheat or simply can't handle the speed. You can try running those games in lower detail or use a older driver to see if that fixes the problem.
edit: another good idea is to make sure the card's fan is working properly, if you already haven't checked that.
edit: another good idea is to make sure the card's fan is working properly, if you already haven't checked that.