Turning the heater on/off makes a difference could mean air trapped in system, are you having any coolant losses issues?
When gauge is indicating overheating, are the cooling fans working then?
Did you try a diagnostic scan to check for trouble codes? If the coolant temperature sensor is faulty, it could leave a record in the engine control module.
If the ground circuit for the coolant temperature sensor or gauge is poor, it can result in the phenomena too.
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Friday, December 1st, 2017 AT 7:50 PM