If the ECT sensor, located on the head next to the coolant neck, was bad then you would have a CEL along with idle problems and possibly hard starting.

The Temp sensor for the Gauge is directly next to the ECT sensor, its the smaller of the two which looks similar to a headphone jack when disconnected. You can determine if there is a malfunction with a volt meter.

Sounds like there might be air in the system though. Remove your radiator cap when the car is cool, start the engine up and allow it to reach operating temp. and the thermostat to open. Top it off, and fill the overflow bottle, then replace the cap and turn the car off.