What is causing loss of power and/or loss of acceleration?

Have a Ford E150 Club Wagon 351 Engine 5.8 L V8 VIN: H. At 45 mph hesitates or it fails to accelerated or will go faster then suddenly drops speed And then holds at 45 mph with equal or more gas application but won't increase speed unless it back fires. Once it back fires instantly it'll speed up. However, in short time drops speed again. Has lost acceleration and lowered mph running between 25-35 mph for 7 miles then back fired and accelerated after up to 45 mph. These are the repairs: fuel injectors power flushed, throttle body and idle air control cleaned. Replaced: fuel filter and air filter, spark plugs, spark plug wires, distributor cap and rotor, O2 sensor, ignition module, fuel injected regulator, fuel pump, MAP Sensor, ERG Valve Selenoid, muffler system and catalytic converter. Back firing has been much less upon new ignition module. Once the MAP sensor and EGR selenoid were replaced, cleared codes and then the fuel pump showed lower pressure and was replaced. Yet, drove several miles and then the same loss of acceleration and power happened with engine light coming on for the first time. Previously, engine light did not come on. Waiting to hear what codes show from the engine light. Throttle body sensor has been checked, honed ok. Any ideas on the cause? Thanks for your time and consideration in this matter! Zwheelin