Temperature swinging

Could be an early sign of head gasket blow-by.

possibly air in the system?

Temperature sensor is the first suspect. They are cheap but most always hard to replace. Do it. It will control the electric cooling fan(s). Next, on level of cost, is the radiator hoses. A collapsed hose could cause this. Lastly, replace the radiator, given this is a '92 model and the original, it is corroded. I doubt it is air in the system, from a head gasket failure or any other means. That would cause immediate overheating and you would see other symptoms from what you describe.

Nope. Early signs are only visible when the engine is revved for a long time like highway driving. Or just a dribble around town. The cooling system can take care of the additional heat, but will show from time to time on the temp gauge. Simple test is to remove the radiator cap, and manually rev the engine while under the hood. Bubbles that correspond to the revving are a pretty good sign.

I agree with Jim on this one. Possible head gasket going bad. See if you can get an automotive HC (hyrdocarbon sniffer) to use in the radiator reservoir. If you know someone in environmental business you could use that type, also. Don't contaminate the probe though. Run compression and leakdown tests.

Some good points have been made. Thanks to all who have responded.

In all honesty fear of another head gasket failure was lurking way back in the dark recesses of my mind. Maybe it was just wishful thinking that this wasn't a possibility.

I'll try the bubble test first as it is easiest and cheapest.

By the way if there is a slight head gasket leak >>

My observations are that the coolant gets murky... less transparent and more translucent, but not really darker. Probably due to suspended HC particles.

Filled up the cooling system to the brim of the expansion tank today and raced the engine for a while with the cap off. Saw a medium bubble of something exit on two or three random occasions. My thinking is that a very small quantity of air might have been left in the system after the recent draining and refill for the Tstat replacement.

Never did see a stream of bubbles of any kind even at high RPM.

Took the machine for a spin with the "radiator" cap loosened. Pretty much the same temperature swing as before, but maybe a little less perhaps because ambient was about 15 degrees colder today. After 5 or 6 miles tightened the cap and headed back home. Noticed the swing seemed to be lessening so drove across the bridge to Ohio and continued driving another 5 or 6 miles during which movement of the temperature gauge needle became barely perceptible. Took the exit and swung back for the trip home. This time the needle movement was noticeable but by no means as much as it had been at the beginning of the drive.

The behavior today put me in mind of a control system taking a long time to reach equilibrium after a step change in conditions. It didn't get much of a chance to reach a steady state on the drive from Delaware because of the ever changing terrain between here and there.

My next move is to track down a sniffer or other tester to find out if there really is combustion gas in the coolant, although it's hard for me to believe there is a head gasket leak after today's experience. Still, an explanation for what is happening would be preferred even if it turns out to be bad news.

Thanks for the update. Paul