I have not seen enough on your post to know but did you just over haul your motor and if so just a thought did you by any chance change your temperature sending unit.We had the same problem on ours as it would show hot in about 5 minutes idling so we put a heat sensor gun on the radiator, engine and hoses and found out it was normal 160 to 165. We changed the sending unit that matched our gauge and it solved the problem.