My Mazda MPV has developed a problem that to.

Hi My Mazda MPV has developed a problem that to myself and the powers that be around me is a mystery. My wife had been using the car in Mississippi while there for a couple of months so im not sure if there was about circumstances leading to my discovering issue. When first checking it out it crank with maybe a little issue but would crank and sit and idle but when you put it in drive it has absolutely no power. To the point you would have to drop it into low finally it would gain enough power to go but pulling a him would be a chore. I put new plugs and wires on it witch didnt help at all. Then decided id take it across town to a friend of mines shop. By the time I got it there it had started dieing when slowing to stop and lost even more power. My friend suggested it was the cats stopped up so we took them off and it appeared they were in deed stoped up. We knocked the gutts out from the two very small ones built into the exhaust manifold and found the main cat to be fine. When pipes were back into place the car still somewhat hard to crank but idled smooth and when reving the motor no hesitations so it seemed problem solved until test drive revealled still no power although ran a little better. Now im not sure if the problem caused the cats to go bad or the cats caused the problem. There is a couple of symptons that I believe will clue in someone that is more knowledgable than myself. Ok when turning the starter over when it hits and starts to crank it will stop turning locking up briefly as if its pressured up or vapor locked. We had to put a electic jumper on it and hook a another truck to it to get it to spin through to start. When it starts it idles and revs up fine but when idling air keeps burping back through the air filter. Not really a backfire but seems like it would be if the air had more pressure behind it. The motor dont seem to have the flow it needs like its conjested. Only other thing I might add would be that the cats before taking the cats off there was very little air from exhaust and now it seems to be plenty so I dont believe its on the exhaust side. I would appreciated any help you could give Thanks