When I first built the system, I had a problem with an annoying blue line above the picture during playback.
I searched for the problem and found a command to change the colour of the line to black, xvattr -a XV_COLORKEY -v 0
I added this to one of the mythtv startup files, but during one of the updates it got replaced. So we have been living with this annoying blue line for longer than we should have. I have been asked a few times to fix it and haven’t got around to it, oops sorry Jo.
Anyway, using the suggestion here http://www.mythtv.org/wiki/NVidiaProprietaryDriver, I have added it to the X11 configuration and now the problem has gone away again.