I've had that occur - it was a bad thermostat. It would stick very slightly open. At idle, or in city driving, it was not noticeable. However, on the highway, the gauge would start to drop and the interior of the vehicle would get colder.
The thermostat I pulled out was probably original, but wasn't stuck. After replacing the timing belt and thermostat, I filled the car back up with coolant and started it up. It did come up to operating temperature quicker
I replaced mine with a low temp thermostat and low temp fan sensor aswell as the infamous blue coolant temp sensor a week ago and everything ran so much cooler and smoother now. My coolant temp sensor failed for weeks and wouldn't run the low speed fan and the water temp was 30-40 degree hotter than usual. Now it's all good with AC blasting in bumper to bumper traffic for 1 hour and it water temp reads 205-215, but my oil are in the 240's.
The thermostat and temp switch are cheap. To clear up a misconception, your vehicle will come to operating temp if you use a 'low temp' unit.
The current plastic thermostat looks a little more beefy, but is still shatty plastic. They're commonly failing at 30-55k miles in the stuck closed position, instant overheat.
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