Red "rash" on plasma TV

When the TV signal (HD cable with set-top box) begins, or when DVD is played, there is a red "rash" across part of the screen. Although the "rash" is generally in the same are, it is not always the same size, and, sometimes, the "rash" is blue, or blue and red. Regardless, the "rash" is mainly noticeable in black areas. There is no "rash" b/f the cable box or DVD is turned on. Dying plasma? Hot pixels? Bad signal?

TIA for any help.

Mike