I have a 2007 mustang v6 4.0 and have not been able to fix it.?

I've changed the maf sensor,the fuel pump, the tps sensor,crankshaft position sensor,the egr valve, spark plugs,spark plug wires,and the fuel filter but still does the same thing. The car runs okay at times and when it fails it won't gain speed when you press the gas pedal you have to press it for a while so it will work at times . It also backfires a little not a lot and not that loud either. I've also cleaned the throttle with carb cleaner. The scanner won't pick up the problem it will pick up the stuff i've replaced. Please help any advice will be helpful thank you.