Picture problem After the set is on for about 15 or 20 mins the picture will dim, sometimes it will brighten on its on, but if I turn the set off for a few mins the brightness returns. There is no audible sound associated with the picture going dim. Is this a sign that my lamp is about to go?

I have an older Mitsibutishi 52" HD 1080. I recently upgraded my cable box to DirecTV HDTV. When I change to format to view in 1080 the TV goes from full screen to what seems to be wide screen with a black bar on the top and bottom. If I change the format to 480p I get a full screen; but the picture has a "redish tint" over it as if looking through sunglasses...I tried everything in my TV manual even call the cable company. Any suggestions?