There's 18v at TP8. Changed cftbl driver board over to Indy (what a mission!) and it tested fine in the single lamp test.

Yeah! I hear you but at least you can focus entirely on the field wiring.

Been thinking, Are the faulty lamps the BC type? Maybe a throw away but I have heard of the insulators breaking down on switches and causing problems so maybe it can happen with the lamp holders too. Maybe it is not showing as a true short on a multimeter.