Backfire/hesitation/sputtering/loss of power

I have replace the coils, plugs, wire, injectors, main wire harness, throttle position sensor, mass air flow sensor, computer, ignition control module, fuel filter and fuel pump, crankshaft position sensor, harmonic balancer in the last 20,000 miles. Currently having an issue where I have a loss of power, under more strenuous acceleration the car sometimes hesitates, pops (backfires). Usually I get 24 MPG now I am getting between 18-20. I drive the car everyday. I notice when I put in premium fuel the problems happens 90% less. But this is eating away at my pocket, I fill up every two days.