bad fan motor, bad fan relay or bad fan temperature switch. Jumper the fan to see if it will work. Change the relay and see if that fixes it. If you need to change the temp switch after all that, go ahead. They are about $40. The relay should be about $15 and the jumper wire- you must have a piece of wire laying around somewhere. If the car cools down while moving, but gets hot when stopped, that shows that the radiator is able to do its job.