Temperature gauge goes down after driving awhile and now rpms seem to go up and down for no reason.
Heat and fan seem to be working fine.
No antifreeze leak.

Hi gramztcg and welcome, you'll get a better answer if you post more info. Good luck

. The Ignition Control Module (ICM) may be failing.

. The radiator's passages may be partially blocked by mineral build-up (phosphate scale). Replace the radiator and refill with 50/50 mixture of antifreeze and distilled water.

Has this only begun to happen in this cold weather of late? If so, you may need to have a slightly higher temperature thermostat installed. AS you drive, the coolant may actually begin to cool in this colder weather. The varying of the rpms could be the temperature control of the engine compensating for the cooler engine. If you notice, when the weather is cold, the car idles faster on startup than it does at normal temperatures(60's and up). That is controlled by the temperature sensing circuit in the system control module (ECU). I don't thin it is a problem unless the temp gauge is dropping all the way down to cold.