I recently had an aftermarket temp gauge in one of my cars go 'nuts' above 190-200 and up to 230, while the engine still was purring at 180-190 (IR-measurement).

Flushed and acid treated the radiator to remove some scaling and now the temps are around 170, or 185 with long idling.

You can always also check an IR-tempreader by pointing it at boiling water.