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What driving conditions was this occurring at (freeway, cit, etc...)? Did you verify it was actually getting hot (boiling over/steaming) or just going by the gauge?
Huge fluctuations in the gauge reading like going from full cold to full hot pretty quickly usually make me think something is wrong with the gauge, wiring, or temp sensor. An engine can only heat up or cool down so fast.
Another quick check would the coolant level. If the level is really low, and pending the position of the temp sensor, it can actually go from reading the coolant to air pockets.
If the temp fluctation is smaller than I would be suscpicious of the thermostat.
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