1991 Chevy Cavalier Engine runs fine, then lean, then fine

Engine Performance problem
1991 Chevy Cavalier 4 cyl Front Wheel Drive 8900 miles

I think I've spent more trying to repair this car than we did on the damn car. We bought it at a used car lot in February of last year and it ran fine, and then about June we started having this problem. You would start the car from cold condition and get about as far as 2 to 6 blocks, stop to get into traffic, and press on the gas and the car would barely want to go, and sometimes flat out stall. We replaced the distributor wires, spark plugs, EGR Valve, MAP sensor, throttle position sensor, fuel filter, fuel pump, oxygen sensor, and finally the catalytic converter before the car stopped working entirely. My father in law towed it to his house and put a new starter in it, which the car would start but still had the problem. He took it to a mechanic up there who read the computer codes and replaced a sensor. (I don't know what sensor was replaced unfortunately) They said the car was working rough but fine and brought it back. The first day we went to get the car emissions tested we encountered exactly the same problem as before, so clearly the mechanic didn't fix it. I'm not exactly sure who besides God at this point is capable of repairing this vehicle, but we're running out of vehicles. The only other thing that I can say is that when I replaced the spark plugs one of them was completly soaked in oil. The salesman had told me the head gasket had been recently replaced, and it appeared there was minor oil burns coming from the top plate of the engine. (I don't know what this is called, sorry) I have checked since and have not seen this same oil leak since. I'm at a complete loss as to what could be the problem here.