I know the thermostat is operating right because when it was warm out and I'd run the engine I could watch the water temp hit 180 and then drop slightly right after it opened. Then it would go back to 180 and hold.
Today I think the water in the radiator was cooling off so much that when it opened it dropped the temp way down and then it would have to heat up again.
It was kind of back and forth between 180 and 160.