Yeah, I agree with the previous poster. It does sound like there's air in the coolant. The temp sensor is at the high point of the cooling system -- where air bubbles will migrate. As the bubbles move under and then away from the sensor tip, the sensor reports "cool" and then "hot" and then "cool"... so the PCM keeps changing from cold idle (high) to warm idle (low) speed.

Bleeding the cooling system should fix it.