So recently my car has been mildly over heating when im stopped. It doesn't go to the red line but it raises quite a bit. The weird thing is it goes back down if i just rev the engine at 1500 rpm for 5-10 seconds. I recently changed my serpentine belts but they look and sound fine, im confused as to what could be the problem. My fluid levels are fine and i changed the radiator and fluid about a year ago.