Hello and welcome to Bleepingcomputer.
First question I have to ask you is when was the inside of the computer checked for dust accumulation?
I referring to the video card and the CPU.
If you have continued regular maintenance and dust is not an issue, it either appears you have a failing video processor or your drivers are corrupted.
Try updating the video driver first.
EDIT: One last thing to try, try removing the video card from the PCI-E slot and re-install it, sometimes the connections suffer from oxidation, this process sometimes corrects this problem.
If the driver update does not fix the problem and dust is not a factor here, then your video card needs to be replaced.
Please keep us updated.
Edited by MrBruce1959, 16 December 2010 - 01:48 AM.