Mechanic said he did that. It ran normal temperature for a couple weeks until yesterday afternoon. In downtown traffic with A/C on, needle started rising. On the interstate it continued to rise to about 3/4 until A/C was turned off. It *slowly* dropped to normal (just below half) then back up to about 2/3. Once home and engine cool enough, I opened the radiator cap and was able to put 1 to 2 cups coolant in it. Also, there was no more coolant in the reservoir than normal. Later I started it and was able to bleed more air bubbles out. Is air getting in somewhere? Will (what appears to be) a small amount of air cause such fluctuations in coolant temp? He now wants to replace the fan clutch but I'm hesitant and doubtful that's the issue.

The general rule of thumb is if it overheats on the highway, it's probably a radiator issue. If it overheats in heavy stop and go traffic / idling, it's probably a fan clutch issue. So I see where he is coming from.

But, with air in the radiator your overflow tank should have been empty. The radiator cap lets air / excess coolant flow into the overflow tank when the engine is hot (coolant expansion) and sucks coolant back out of the tank as the engine cools.

I would replace the radiator cap at this point - it may be letting air in - and check the little overflow hose that goes to the overflow tank for leaks. The leak can be pretty small so check carefully or just replace the hose.

Fill with coolant, and watch the coolant level in the overflow tank. Check once a week or so when the engine is cold. Pop the radiator cap and check for air.

After that, the coolant level in the system should be stable if all the air has worked out. Then you can see if you still have a problem.

Indeed, it was the radiator. It was replaced (in 2016) and has run at normal temperature ever since. I apologize for the very late conclusion but ran across this picture while cleaning my hard drive.