Car loses power when accelerating

I have a 1995 Chevy Lumina 3.1 liter engine with about 150 thousand miles on it. When you first start the car, until it gets warmed up, the RPM's will drop to about 500 and the car will lose power. If you press harder on the gas pedal the car will hesitate, then the rpms will shoot up, the car will go again and the RPM's will go back down. Also, after the car is warmed, if you turn it off (say, just long enough to get gas) and try to turn it back on it will either not start (it will just sputter out) or start roughly and then the RPm's will fly up to about 3000-400 on their own and then fluctuate for a bit and even out to an idle. I had the car to the shop just today, and it was throwing a code 51. They replaced my IAC, my PCV, my o2 sensors, my #5 fuel injector, the entire exhaust system, my map sensor, and a laundry list of various things. They also looked at my fuel pump and filter and said they were running well. The car sputtered on acceleration as I was pulling out of the lot. I took it back and the mechanic could not reproduce the problem. He said it would probably "even out on its own because replacing the PCV is like resetting the system " and that my car would need to re-learn what speeds it idles at, etc. The car ran great, for a few hours after that, but after leaving work tonight, it is running the same way it always did (minues the service engine light). What *else* could be the problem?