Intermittent

Check the thermostats for a tiny bleed hole with a weight dangling in it. If there is none, drill a 1/16" hole in it. When it is totally closed, it takes a while for the heat to get to the sensing element in the bottom of the thermostat. While waiting for that to happen, the gauge sending unit is already registering an increase in temperature because it is closer to the source of the heat. Once the thermostat finally opens and coolant circulates into the heater core, the cold coolant comes rushing in from the radiator and the thermostat closes again. Given enough time this will even out, but that little bleed hole will let just enough coolant sneak through to warm the thermostat and cause it to open sooner.

Caradiodoc