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What you're describing is consitant with a thermostat that's "sticking", resulting in binary operation (full open or full closed) rather than modulating operation.
You can remove it and test it with a pan of hot water on the stove. (Remeber that antifreeze is poisonous and take proper precautions to avoid accidental ingestion!) You should see smooth operation.
If you have instrumentation available, I would put a thermocouple in the vicinity of the temperature sensor and validate that your temperature gauge is accurately reflecting what the engine is actually doing. This will rule out (or identify) an instrumentation issue vs. a thermostat issue.
Roger
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