couple of good ideas there check them first
I think what happens is that the video and audio goes through small buffers and are then sent to the screen in a stream - ach new frame forces the last one out of memory
because aud and vid are seperate if either feed goes missing - the memory hangs on to the last bit of information
if you unplug and leave - when you switch on again the picture will have gone leaving just audio
occasionaly my telly does this - by switching off and on then opening the sky prog guide the pic comes back
it only happens with the digital feed - the analog option never does it