Everything ELECTRIC all new, run,s great then goes to crap, runs great then crap

My first instinct is to revisit where the problem started, the used fuel pump. While thinking it over, I wonder if the circuit for the fuel pump was damaged while the bed or fuel tank was reinstalled (depending on which you pulled). Also, perhaps there was some gunk in that fuel pump that is now causing intermittent clogging in the fuel pump, filter, line. You should be able to pull the bed or tank out, run a couple hoses to the pump and diagnose it as a unit with key on engine off, and key on engine on. Pressure test before and after the filter a few times to see if you can get it to clog, test the circuit, flush the lines.