while on a cross country road trip, the truck started to feel like it lost gas or a strong tug, on and off (the tug) for about 30 seconds. then it started to over heat alot!!! we went to a shop and they installed new spark plugs, cables, oil, and air filter and cleaned out the radiator. problem persist i took it to dianostic (what ever the machine) and every thing appears to be in perfect condition WHAT COULD IT BE?