It seems that whenever it is in the VIDEO mode there are black vertical lines across the TV. They are worse on the right hand side. <snip> It is hard to describe, but they are thick, translucent lines

You seem to be contradicting yourself a bit here but translucent (with a darkening effect) is what I think you are are getting.

On that assumption, it's probably a phenomena called Ringing. What may be happening is the AV signal is being reflected up & down the AV cable and contributing to the picture each time on each reflection cycle, it will eventually diminish to the point where it doesn't show up on the screen. I.e. it's less noticeable further from the edge of the screen.

This could be caused by a fault but it's probably an impedance mismatch between the equipment or a poor or incompatible cable.