Engine temp problem

Very often there is a small bleed hole in the thermostat to allow warming coolant to get over to it. The sensor for the gauge is a ways away from the thermostat. By the time the heat migrates over to the thermostat the gauge is already showing the temperature is too high, then when the thermostat finally opens, cool coolant rushes in from the radiator, the gauge goes back down, and the thermostat closes again. Eventually it stabilizes but it can cycle like that for 15 - 20 minutes. If that bleed hole doesn't exist in the new thermostat you can drill a 1/16" hole through the middle of it.