Temp guage bouncing up and down just above half way point.

Temp gauge rose way up, then came back just above half. As it drove down the highway, the gauge bounced up and down. Checked under hood, there are no leaks in coolant system and the reservoir is at proper level.

Thermostat?

I am going to try that first. I'll post results.

Replaced the coolant temperature sensor and that solved the problem. 15 minute repair.