You mentioned cable box.  The Scientic Atlanta boxes (SA 3250, 8250 and others) have an aspect ratio - read, "zoom"- built into them. You have to really dig into the remote control documentation to come up with this--->  the pound sign (#) next to the number zero (0) is the toggle for the zoom function.  Just press it repeatedly to go thru all the possible sizes.    It is, unfortunately, easy to accidentally trigger the # key by holding the remote in your right hand and using the number keys with the right thumb to direct access some channel ending in 0, like 50.  A big enough thumb may accidentally press the # key and trigger a zoom change. Stupid design-yes!  May be nothing is wrong with the TV.