On Demand Picture Blanks Out

When watching anything on demand, the picture turns black and the sound stops.   This happens when viewing HD or SD, movies, tv shows,  premium channels or regular Network channels.   It started about a week ago.   I have to press stop on the remote and then restart whatever I was watching.   And,  yes, I've unplugged and reset the box a few times.   Any ideas as to what the problem is?