Every once in a while, but more and more frequently lately, while watching CBS over the air I get a black rectangle that takes up about half of my TV screen and shows either greater-than signs that keep getting added, one at a time, until the entire top line of the rectangle is filled, and then they stop, or the word "CBS." If I change channels for a few seconds and go back to CBS, the rectangle goes away, but only for a brief time, and then comes back. This does not happen on any other channel that I receive over the air.