you asked him to replace it without getting the cost of him doing the work. A new radiator cost me around $160 and took about an hour start to finish to get in the car. Before I have a mechanic do anything I read this board and look at what it takes to fix it myself. Most stuff I have been able to do and when it was more than I could do I had to bite the bullet and pay someone else to do it. There are many threads on this board about bad thermostats. If your car was not running hot before the thermostat switch then I would suspect the new one was bad and try another or at least check the new one before installing it in the car. Also what are you calling running hot? In the red zone on the gauge, or just hotter than what you were used to seeing? After a new thermostat my 300 ran just a hair above the 80c mark. In the dead of summer in town or on the open road at 75-85mph it gets close to 100c. Seems normal to me and as long as it does not get into the red or start eating antifreeze I will still call it normal.