Jeep wrangler temperature gauge

I have a 4 cylinder Jeep Wrangler. When I drive, the temperature guage will rise steadily to about 3/4 of the way up, and then drop back down to below 210. It does this pretty consistently. It never gets to the orange, and never overheats, but I’m nervous it will eventually. I topped off the reservoir and the radiator was full with clean looking fluid. There is some corrosion or buildup on the top part of the radiator. Any ideas what could be causint the temperature to fluctuate like that?

The problem might be with a thermostat that’s opening late. Have you tried repalcing the thermostat?