Why does the gauge needle of the car moves up and down, showing normal temp sometimes and high temp sometimes?

The car gets hot sometimes. Yesterday I heard boiling coolant under the hood. The needle goes up and sometimes stays up, but sometimes gets back to normal. The mechanics are not sure what the problem is. One of them wants to guess. He wants to change the thermostat.
Why does the gauge needle of the car moves up and down, showing normal temp sometimes and high temp sometimes?
starting with a thermostat is not a bad idea
Why does the gauge needle of the car moves up and down, showing normal temp sometimes and high temp sometimes?
Go to a mechanic, this one's a parts changer.

If it was the thermostat, the car would overheat in 20minutes and it wouldn't cool down.

Not knowing how the water pump is located for whatever car you have, I would think this has something to do with the water pump, going bad and stalling.

Or the radiator fan(s) are not working properly.

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