+ 1 thermostat.
When engine is cold, then all water circulates around engine. As the water gets hotter then the thermostat opens. Then water circulates through engine and radiator. If the thermostat is playing up then you would see ...
Engine start - indicator shows cold
Engine warming up - Indicator shows warm
Engine hot (thermostat still closed) - Indicator moves towards hot
Engine hot (thermostat open) - Indicator moves towards cold as radiator water enters system
This is pretty much what your seeing.
You could remove the thermostat and then try to test it out of the vehicle by putting it in a pan of hot water and then heating it. You should be able to see it open.
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