Just wondering if anyone has come across this problem?

The lines are very noticeable, and look a bit like analogue teletext.
There are about 6 lines, moving quickly down through the picture.
They appear on RF and AV inputs, but NOT on the HDMI input.

Effect can be seen here http://www.youtube.com/watch?v=eTfduCtARAs

I get pictures ok, I just used a blank screen in the video to show the
effect of the rolling bars better. The TV is about 3 years old.

Anyone know what might be causing this?

TIA

--
Bryce Whiteford