1992 Mazda MPV No power and not starting

Engine Performance problem
1992 Mazda MPV 6 cyl Two Wheel Drive Automatic

We have a 1992 Mazda MPV. We purchased it used with no problems at first. After about two months, the van starting losing power and wouldn't hardly make it up the littlest hill. We did a complete tune up by replacing the rotor cap, plugs, wires, fuel pump, timing belt and fluid changes. The van worked just fine afterwards and almost ran better than it did before. About two weeks ago, it started acting up again. But this time, when it went up a hill it would lose total power and wouldn't go unless the pedal was down to the floor. We went outside to leave three days ago and the car wouldn't start at all. We then checked the timing to make sure it was good, and it was. Check all the fluid again, and the plugs. They are all fine. The codes on the car, when it was running, said it was the O2 sensor. But two mechanics said that that alone wouldn't keep the engine from starting. It just acts like it is starving for fuel so we did a pump check on the fuel pump and it was also fine.

We are just about to give up on this car. Any comments or answers would be greatly appreciated.