It could be what I call 'rebound'. On computer monitors you can adjust the vertical and horizontal positioning: two far in one direction, and it would 'rebound', with a faded, reversed version stretching back across the screen in the appropriate direction. The TV equivalent could be that only the very top of the frame is being warped this way, and on certain channels you get black spacing at the top and bottom, hence any warping would be there, but invisible. Your best bet is to have a TV shop look at it, but it's probably going to cost an arm and a leg.