Sounds like the problem with the monitor that came with my first system. It's an HP M50, also 5 years old, and a couple years ago it started flickering out and the whole screen would go purple (Hsta, you old Hydran, you'd love that!), like the green gun was going p00py. But get this: Since I brought it home (as it was being used by my sister), it hasn't acted up even once!
It is still going bad though. When there's a lot of light colors on the screen, like large open windows or bright web pages, only a 3inx3in patch in the middle is readable at 1024x768, the rest around the outside is blurry. The screen blurs at any resolution, but 800x600 is readable all around.