I have an 04 elantra gls. was overheating so i replaced the thermostat. After replacing the thermostat thought the problem was fixed. Wife took the car the next day and after driving for a while the temp went up to about 3/4 and AC got warm. However, with the car just idling temp goes back down to normal. I checked the antifreeze level and it was normal so nothing is leaking. Any thoughts as to what it could be?

 

Check the fans and the water pump.

 

fans are working. As far as the water pump, i don't have any antifreeze leaking anywhere so i don't think the water pump bearing is bad.