2003 Ford Expedition Engine hesitates during acceleration w

My Expedition hesitates during acceleration when the engine is warm. It feels like a misfire. I have replaced the plugs, carefully gaping them to.54 as required, replaced the fuel filter, have cleaned the throttle body and have run a Lucas fuel injector cleaner, and run 93 octanes every 3rd tank. I suspect my fuel pump is going because I hear an intermittent knock from the rear of the vehicle, where the fuel tank sits. The original plugs were replaced at 130 K, when the problem began, and I just replaced them again 200 miles ago, because the problem re-surfaced. The original plugs were heat-range 7a, the ones I'm running now are Motorcraft 9c heat range. The hesitation only is most noticeable when the engine is between 1000 and 2000 rpms, and the engine runs fine at a steady speed, but will hesitate at any speed when I step on the gas, no matter how slightly I do it. I will appreciate your advise.

Thanks!