Yet - thermostat. The dealer wanted $122 just to diagnose the problem. They said if it was the thermo it'd be at least 1hr labor plus parts. Total would be at least $250. I put a new thermostat in in less than 10 minutes. Under $40. I can't thank y'all enough for the advice.

Is it a reasonably easy thing to flush & fill this car d.i.y? Or should I leave that to the pros? I'd like to take care of it just to make sure it's done now that everything seems good.