Go into the menu and see if your closed captions are on and set to "TEXT". Setting the closed captions to "OFF" should solve your problem.
Suddenly a black box pops onto the lower portion of the screen. If you press the volume on the TV remote it goes away but will pop back on sometimes right away other times after a few minutes and sometimes, on a good night not at all. Also goes away if you go through the screen formats until you get back to standard.