I was gonna say to check your cable first, since I've had the exact same symptoms before, and that's usually the culprit. The next thing that sometimes causes it is when one of the guns in the CRT starts to go, leaving only 2 of the RGB's 3 to 'paint' the screen. (What can I say, I had bad luck- the monitor that started out with a bad cable got replaced under warranty, but then the replacement went bad out of warranty with similar symptoms, but not the same cause.
But since you say the OSD isn't affected, I'm stumped.
Edit: Sitting here thinking about it, I still say check the cable. It's not the tube if the colors in the OSD are fine, but that doesn't rely on the signal from the PC end. If the cable's loose or otherwise going bad, you could get those same symptoms. Could also be the video card itself..