I gotta make a 1500 mile trip tomorrow and my truck's overheating. Took out the 160 stat and replaced with 180, replaced fan clutch. flushed radiator with the water hose for about 30 mins at running temp, and it's still overheating. I don't know what else to do. Not leaking anything either. It's getting up to 235 maybe a little past then it'll drop back down around 190-200. then it'll go back up a few mins after that. If I run anything over 2000rpms it starts getting hot and stays hot. The overheating doesn't start till I get on the highway at a constant 70 mph. any ideas???