That could very well be a cooling problem, as others have mentioned. I just posted
here how to check your temps with a simple tool. The heat sink could not be seated correctly or the heat sink fan could be not spinning at all. If that's the case, get it replaced.
If the temps are normal (for that card, they shouldn't really exceed 60*C), I'd see if any other GPU-intensive games have artifacting, as you could have a defective GPU or memory. If other games are normal, try reinstalling SHIV.