Akai CFT2726 - Black Box hides Video

I have an Akai CFT2726 (Walmart purchase 2005) 27" flat-screen CRT TV.
Sometimes a black rectangle the size of the setup menu appears and
covers the video. It removes when you hit the "exit" button on the
remote, but quickly reappears. It looks like a mask for the setup
menu, or a blank menu. The TV uses Konka remote codes, so may be of
their manufacture.
Help appreciated.