Do you have a speaker next to the screen? The magnetic field generated by that would be the cause of the problem. Also, if your pc is next to the screen, the magnetics from the hd/psu could be the culprit. And if you have a fan on or near your desk, that could be a problem.
If you feel that this is completely unrelated to your problem, then oops.
But my old video card (fx5500) would literally paint the screen green at random moments throughout the day, its favorite times included in the mos timprotant battle of a game, the climax of a game, or when i was about to save a project. It looked as if someone had a huge green brush, and it would do random up, down, and diagonal motions until the whole screen was green. So i got a new vid card. Now its fine.
Intel Conroe e6750
SLI EVGA 8800gts KO's
4GB Patriot DDR2 800mhz
22" Widescreen Sceptre LCD monitor 5ms
OCZ 700W psu
320gb seagate w/ perpindicular tech.
Creative SB X-Fi Fatality
Thermaltake Armor: Extreme Edition