Hi Rob,
Tricky one. Could say change the main board, but might I suggest you enter the service mode, click on 2nd item in the menu (CONTROL), and scroll down till you locate WD COUNT [Watchdog Count]. If it reads any number above 0, main board should be replaced, but prior to doing that, do a factory reset by returning to 1ST item in menu (OPTION) and locating factory reset. Re-enter the service mode and you'll find WD COUNT has been reset to 0. Then locate WATCHDOG, and set it to OFF. See if the TV behaves after that.
Bruce