I forgot to mention that I actually removed the LCD module completely whilst keeping the pcb cable and inverter supply wired up. I then replaced and reattached the LCD "frame" so that I could use all the buttons if required, although in the event there was no need to do so.

With the LCD in my hands I can make the fault appear and disappear by applying a very slight twisting force to the module. This could well be a loose joint somewhere but I'm very unsure about dismantling the unit any further even though it looks like the thing is held together by high strength adhesive backed felt tape. I dare say duct tape would hold at a pinch when reassembling but I have no idea what lurks within so for now I can live with the fault.

The pcb cable seems secure at both end, I've given it a waggle and the display is unaffected.

The video is output from the right hand part of the main motherboard that occupies the shielded frame which is the top of the pcb stack once you get the bottom off the keyboard. There's no sign of any electrical stress in there, you'd think it was fresh from the factory to be honest.

Having the schematics would help, could I get these off Yamaha?