It just started a few days ago. It takes up about the bottom half of the screen with a little showing on the sides. It's a Sylvania 4273

It just started a few days ago. It takes up about the bottom half of the screen with a little showing on each side. It's a Sylvania 4273 - pretty basic - no PIP or split screen. When I change channels the screen is OK for a short time (less than a minute) then the black rectangle appears. It's there during commercials and on every channel (I have comcast). Repair or replace? Or , is there a fix?

Thanks.


THIS IS NOT AN ANSWER! I didn't finish the question on the first post.