I just replaced the radiator, thermostat, exspansion tank and cap, every hose possible ex. The truck runs excellent until I speed up to over 50 miles an hour. I can let idle and drive around town but as soon as I get on the highway the guage jumps up GRRRRRRRR. What could this be???? I'm pretty sure the air is out of the system. I just don't get it, any help would be much appreciated..thank you