Thermostat, first thing to check. They're cheap and easy to replace.

Anecdote: Driving home from the airport after a week trip; 4 miles form home notice the temp ind in the upper middle of the range and heading North fast. PUlled over and checked the upper hose. Tight as tick and mucho hot. Engine didn't get preigniting hot but was on its way.

Waited 1/2 hr to cool down. Did this once more before making it home. Next AM have to go to work, so remove the vaunted Robertshaw 195 less-than-two-years-old and stuck in the old 180 one I (fortunately) hadn't discarded. started and ran fine; back to normal temps on the way to work.
PIcked a new thermostat at Carquest on the way home. "Tested" the Robertshaw in a pan of water and it seemed fine, but it surely was the cause of the overheating.

My $0.02