Engine lacks power

1997 f-150 258,000 miles but rebuild at 135,000. The problem started a couple weeks ago. Sometimes when I would acllerate I had to put the pedal all the way to the floor in order to get any kind of momentum. I took it to the ford dealer and they said it was the catalytic converters. Replaced them. Runs somewhat better but still lacks power. With the ac on it really bogs down. I have replaced 2 of the o-2 sensors the maf. Also when you race the engine in neutral and let off the gas it will backfire. I know the dpfe needs replacing. But the ford mech said that would not cause my problem. It runs the same whether the shorting bar for the pcm is in or out. When this first started it would only do it for a second then run fine. Any advice would be greatly appreciated.