Intermittent

Check the thermostats for a small bleed hose with a weight dangling in it. If there is none, drill a 1/16" hole in it. It takes a while for the heat to work its way to the sensing element in the bottom of the thermostat. The sending unit for the gauge is already registering an increase in temperature because it is closer to the source of the heat. When the thermostat finally opens and hot coolant flows into the heater core, cold coolant comes rushing in from the radiator and the thermostat closes again. Eventually this will settle down and stabilize, but to make that happen right away, that bleed hole lets just enough coolant sneak through to get the hot coolant to the sensing element sooner.

Caradiodoc