Quote Originally Posted by petrified.rabbit View Post
that could also be the instance the thermostat opens and the cold uncirculated coolant in the radiator gets added to the system.
But after it opens he shouldn't continue to see unusual fluctuations on the gauge. He said that "the engine was completely warmed up prior to this". If after driving for about 30 minutes you still don't have hot air coming out of the heater, it's a defective thermostat. "Quite warm" is not the same as "hot" - but only he can be the judge of that.