Hi Ian,
I have had similar faults on other makes and it usually turns out to be the "screen" driver PCB which is usually part of the LCD panel (it is sometimes called the LVDS reciever)
Try waiting for the set to go in to fault and then feed the video signal out of the scart socket into another set, if you have a picture on the 2nd set then it probably is a faulty screen driver board, if you dont get a picture on the second set then it could be a fault on the main board.
Its worth asking the manufacturer if they supply the Screen driver board as a seperate item(JVC and Toshiba do on some of their models)
Hope this is of some help
Brian