Horizontal color distortion on Mitsubishi WS-55819 rear projection HDTV

I've had my Mitsubishi WS-55819 rear projection HDTV since 2002 and in general it's been a great monitor. Just recently however, I've noticed a constant effect of a purplish horizontal band that keeps appearing at the bottom of the screen and moves upward and then repeats. I have several devices attached to the TV, including an Xbox 360, PS3, and DVD. I just recently bought an A/V switcher and the problem is just as pronounced (even more so) when I use these devices. But the effect was present even without the A/V switcher and I've isolated the problem to the TV itself. Any ideas on what this problem might be caused by? I'm dreading the possibility of having to get a new TV (as much as I wouldn't mind it) so any insight on what might be causing the problem would be great!