Hello Nick: Not sure the year of your Plus 4 but I would check for two things, poor earth connection and faulty voltage stabilizer. As voltage stabilizers are cheap it is worth just replacing it, assuming you have one fitted.
Well at the MOA Cocktail Party in Sun Valley I bragged that I could get answers to any Morgan Problem! Wrong! As My informants have no or little knowledge of the Newer Morgans I don't have a clue. About 1970 is My limit.
