No power and backfires on acceleration

Cranks and idles fine. New water pump, thermostat housing, new thermostat, catalytic converters, new o2 sensors, new plugs and wires, new fuel filter, complete engine flush, radiator flush, new belt, new battery. Even took it to Ford dealership to figure it out and all they wanted to do was put a new engine in it wouldn't tell me what was wrong. So I took it to another place to have injectors checked out working fine they even replaced. All sensors one at a time to see if one of those was the problem.

All that has been done truck will still only go 20 MPH most of time and starts back firing, when it runs ok can't get to 2,000 RPMs or it basically shuts down to 5 - 20 MPH and starts back firing all over again have no idea what in the world is wrong. Help please my only transportation.