Temp guage erratic in 96 Camry

Hi, I have a 96 Toyota Camry, 4 cyl, A/T. There was not much heat from the heater and the temp guage stayed on the very low side. I replaced the thermostat and radiator cap. The radiator and hoses had been replaced about a year ago. Anyway, it was fine for a few days. Today however, the temp guage stated going up when going up any type of hill, large or small, and would go down when going downhill or level. The movement of the hand would be fairley rapid. It never went into the red but got pretty close. The radiator is full. Any ideas would be greatly appreciated. Thanks

Are you checking the coolant level when the engine has completely cooled down? Is the reservoir also full up to the proper mark at that time?

Are your electric cooling fan{s} coming on when the gage shows HOT?

Some fluctuation going uphill and down is normal. Seeing that happen suddenly isn’t.

Well, all at once it started working properly. I may never know what actually caused it.

There may have still been some air trapped in there. On many cooling system designs the air can purge itself eventually. But if it happens again I’d suspect there is still some air in there.