9800 NP gets bad fragments after a while, but i reboot and its ok?

I have a 9800 NP and if I leave my PC on overnight or for a long time, when I come back my screen is full of fragments, usually so bad I have to do a cold reboot because I can't navigate to the correct shut down menus. When I reboot, however, it comes up just fine. I have great temps so I don't know what the problem is. I do have it OCed to about 410/310, but it seems stable aside from what I've described above.

Help?

sounds like your card is getting too hot.