I've had Overheating problems out of my Nitro for the past week I replaced the radiator, and thermostat. (The Radiator had a small crack.) Well once I got all the air bled out it overheats when driving down the road but if I leave it running after driving it, it cools itself back down. I was told it could be the Highspeed relay switch to the electric fan. How would I check/test it to find out? Also the waterpump is fine and there are no headgasket leaks at all. I have 195k miles on it, and it's a 2007 3.7 v6 2wd.