I have a 2000 jeep wrangler that runs great, except for on a long drive (45min) when tge check gauge light comes on I pull off the side of the road an kill the engine! After about 2min I recrank the engine n the temp gauge starts to decrease temp! I know it's not the gauge cuz under the hood it gets really hot n radiator fluid in resevoir is boiling! Could it be the fan? ( changed water pump, radiator cap, and thermostat is good) any answers would greatly b appreciated