I remember starting a Dell Monitor support thread at Dell.com, because I thought that it was the monitor and nothing else. I tried like 3 different monitors, and finally the Dell customer service guy told me my graphics card was overheating.
Got a new 7600 (since my old computer only took AGP) and it fixed everything.
EDIT: The reason I got a new one is because, after trying 3 different monitors, my graphics card was totally fried by then.