Plasma screen distortion

I bought a pioneer plasma 50" pdp503cmx about 4-5 years ago. For
several months I have noticed a bright yellow area (or green if
something like a golf match is on with a lot of grass) at the top of
the screen. It was very small (probably a year ago when first noticed
it) but has been getting bigger. i have cable and can see a faint image of the chanel discription imposed over the active image. appears to be part of the chanel guide. almost like two channels are comming through at one time. this is a comercial grade monitor.
1. It will go away if I turn off the screen and let it cool down and
then gradually return as it heats up over a period of perhaps an hour or two. I only noticed this recently but feel it has been there from
the beginning of the problem.
2. It is not in evidence with DVD input.
3. It is only rarely at all visable with HDTV and then only very
faintly.
4. it appears to come and go, visible (even after hours of use) on diff. channels. very odd.
i dont believe it to be burn in because of the color and its not at all visible all the time. i have been very careful to not ever let an fixed image stay on for very long. does anyone know what this might be and if it is repairable. i paid approx. 10k for this tv and believe it should not be a loss. i thought it may be something wrong with the cable but have tried satalite and see the same yellow/green hue.