Quote Originally Posted by JDMGRNAV6 View Post
you need to flush the radiator, put new fluid in and let the car idle with the rad. cap off, this allows the car to essentially burp itself, letting the trapped air escape. this may fix your problem.

this same problem happened to me on two different cars
Try this, those little air bubbles get in the cooling system and that's why it gets hot and cool off.....then gets hot again. Some cars have inline valves you can open, if not just do like he said above and remove the radiator cap.