I've recently noticed my dlp tv has this problem. Its pretty much a very lightly shaded horizontal bar that slowly rises from the bottom of the screen to the top and then starts all over again. Its about half an inch thick and spans the entire length of the screen.
Its hard to actually notice this but if you actually focus on it you can see it slowly move up the screen because it tints the picture a slight shade of purple as it moves.
No idea what this is and that's pretty much the best description I can give of it. However if you have ever seen a recording of a tv/computer monitor where the screen flickers its kinda like that only this bar takes maybe 20-30 secs to move the length of the screen. It is very slow.
This is something I haven't hear in a long time. Ok here is what you can do, disconnect all of the input cables (component, s-video, hdmi, etc.), the TV has to be attached only with the power cable. Now turn the TV on, is the horizontal line still there? -If yes, now try to bring up the menu. If the horizontal line is still there then there is a high probability that the DMD board is bad. -If not then the problem isn't the TV, its one of the external sources or cables. Try hooking up the cables one buy one until you find the one that causing the problem. Now we can narrow it to two things, the cable or the actual device (DVD player, cable tv, video game consol, etc.)