Bad thermostat?

Full size bronco 302. I'm getting erratic temp gauge readings. Driving down the road it will be at the normal temp, then go all the way to hot then back down to normal temp. I always thought if a thermostat was bad
it just wouldn't heat up at all. Temp gauge is very erratic.
Thanks Darin
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Could be low on coolant, water pump going out, slipping drive belt, bad radiator cap, timing "off", leaky head gasket, fault temp sensor/gauge, or bad electrical connection.
Darin wrote:

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