Sounds like air pocket(s) in the cooling system, probably been hiding in the heater core. Couple of ways to deal with it. Park the car on an embankment with the nose much higher than the tail, pop the radiator cap and let the engine run for a while to evacuate the air trapped in the system. And/or, loosen the bleed screw on the coolant line on the front/left (looking at the engine from the front) side of the engine to allow air to leak out of the system.
Once you get all the air evacuated your temps should stabilize and not dance all over the place.
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