Resolution problems!!!

My Toshiba 51" Theaterwide HD
http://www.tacp.toshiba.com/televisions/product.asp?model=51H84
was working fine for over a year, now theres a problem limited to when I switch from HDMI (cable box) to a different input and watch a movie. It sounds strange, but there always is a big black box in the middle of the screen while playing a DVD. It doesnt matter what player I plug into the TV I've tried with 3 different players, playing movies I know work on other TV's. So I eliminated the DVD, and DVD player leaving only the TV. There must be a setting that got changed somewhere but I'm having trouble finding it.
thanks in advance for any help.