Take fuel filter off, can you blow through it? If there is any resistance put a new filter on. If the filter is clogged, it will make the pump work harder under driving conditions. Would explain why fuel psi is normal at a shop sitting idleing....most shops dont test fuel psi drving down the road....basic senario, fuel pump works harder than usual. It gets hot starts amping out and eventually kicks the relay. Wait 10 mins truck runs good again, until pump gets hot again. Also. Gas in the tank helps cool the pump, running it low compounds the problem.