1991 Mazda MPV no power

I bought a 91 MPV a few weeks ago after a short test drive, seemed sluggish between idle and 3/4 pedal, but ran strong at full throthle, so I thought that was normal. I had the owner drive it to me for delivery about 50 miles, since I wasnt able to drive two cars at once, and it arrived fine with no overheating or any obvious problems.
After driving it the first couple of times, I became certain that the lack of power at anything less than full throttle was not normal. Also noticed in that acceleration range is a slight but noticable surging (about 1 or 2 pulses per second) until completely warm, then just slow, like driving at 1/8th pedal, that is until full pedal, then acceleration seems normal with no unusual noises or power drop. It starts fine, even in cold weather.
I paid one hour diagnosis at my local repair shop and they confirmed the lack of power within that range, but could not fine anything wrong other than a small (fixed but no change) vacum leak and a disconnected O2 sensor, that when plugged back in, makes the car run terrible. I saw where the mechanic pulled back some of the sensor boots for testing, so I assume he tested things like TPS, MAP, etc, but cant be sure on everything, since it was only one hour. The mechanic seemed sure the exhaust was OK and the mechanical engine was sound. Without putting too much money into it right away, I asked if there was anything I could look for on myown. He said check the ECM connectors and underneath the ECM covers for corrotion. I did and everything looks brand new, with nothing bent over or out of place.
One thing I can point out is, other than the O2 sensor, there is a sort of vacum valve connector on the firewall intentionally unplugged with the hoses bypassed, like someone was looking for this same problem already.
Any feedback is greatly appreciated. Please feel free to let me know if I can help by answering anything additional, or also I can check anything you need and get back to you. Thanks again, John