It sounds more like a driver issue with your graphics card, have you tried updating to the latest version?
What version are you currently using?
Also which input are you using? VGA, or DVI? VGA can go up to high resolutions, but aren't really supported above 640x480, even though it would work, it wouldn't work optimally.
It could possibly be even your RAMDAC that might be going/screwing up if your using VGA.