mikeru

I suspect you're right about the gauge moving towards the normal range before dropping down. But the only other thing I can think of, assuming the thermostat is working correctly, is maybe a previous owner bypassed the thermostat by modifying the coolant hoses in some fashion. I've seen this done as a temporary fix, but only in extraordinary situations.

I've seen instances where a bad radiator cap can cause issues with the cooling system. They're designed to keep the cooling system pressurized, but to allow the system to vent into the overflow if it exceeds a certain pressure. I'm not sure how to test one, but it might be worth looking into. Maybe yours is allowing the system to over-pressurize.