I have a 2005 KIA Rio with 72,000 miles on it. Not all the time but sometimes the temp gauge will go into the hot then come back down to the temp it normal is just at half. This only happens when you drive it. You can let it run all day sitting still and its fine. After parking it and shutting it off the coolant comes out the overflow bucket. The coolant is up in the neck of the overflow bucket. I have replaced the thermostat with one from the dealership. Replaced radiator cap , also had a pressure check done. Everything was fine. The timing belt broke at 72,000 so I have redone the head and all gaskets was replaced then. The belt makes a noise off and on that runs around the water pump. Not sure if that has anything to do with it . We are really confused as to what it could be . I’m hoping you will have some insight .
My first thought would be the cooling system is low on coolant or has an air pocket. This would be a common cause for the temp to go way up and then come back down. (most likely cause)
After seeing you had some head work done. I would be curious to see if the pressure in the radiator goes way up with the engine running(compression getting into the cooling system). You can attach a cooling system pressure tester and run the engine to see if it spikes the pressure gauge.
Another thing to take a look at, your fans. Turn your AC on and see if the radiator fan kicks on. If it does, you may want to see if when your driving and it starts to get hot if turning on the AC causes the temp to come down. I think this system has a high and low speed fan relay. You can check these and the fan motor if the fan does not come on.
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