Well overheating is the most common cause.
Firstly make sure your drivers are all up to date. That may be a simple way to fix the issue.
As for the high temperatures: if you're no longer under warranty, open up your case, and take the video card out. Make sure you ground yourself first (anti-static bracelet if you have one - if not google methods of grounding yourself) as you don't want to damage anything. Once the card is removed, clean it and add some more thermal paste to it. Make sure your fans are all spinning fine and aren't all clogged up with dust (best way to remove dust is bottles of compressed air - DON'T use a hoover.) Once this is done, put the video card back in, and wire it all back up. Then try again, and see if the artifacts are gone. This is by no means guaranteed to fix your problem, but it may help. Be careful - if you mess things up more then you've got no one to blame but yourself
Just make sure you put everything back exactly as you found it, and you'll be fine.
Have you overclocked? That may have caused this issue if you have. Regardless of whether you have or not, it sounds like you'll need to underclock to reduce the heat generated by your system if the above methods have failed you. Get Rivatuner to help with this - the instructions are pretty simple. You'll lose some performance at the end of the process, but it's a choice between performance loss or artifacts. As per above, this may not necessarily fix the issue, but if the other methods have failed then you could try this. Again though, be careful playing around with the clock settings, even with RivaTuner. Make sure you make notes of all the numbers, so that you can edge your settings back up to where they were originally should you ever want/need to.
Wish you the best of luck,
-Nih