Car went crazy after being serviced HELP!!!

[mike690003]

Hello Board,

Today I took my car to a well known and very reputable shop to get my air/fuel mixture adjusted. It was previously tampered with and caused my car to run very rich. My car was put on all the specific computers and it showed my car was indeed running a little rich and had a slight rough idle. The mechanic adjusted it to spec and then my car idled smoothly and accelerated great. After reaching close to home(drove about 15 minutes) the car began to run weird. It hesitated really really badly and stalled a few times. I was almost rear ended TWICE because of this. It was as if the car began to run very lean. Well I called the mechanic and took it back to him. He then richened the mixture and now the car pours black smoke out the exhaust when I accelerate (like before).

Why did the car react negatively when the mixture was adjusted to spec???

I have replaced:
Cap and rotor
Plugs and wires
Air filter
Cleaned idle control valve
All vacuum lines
Fuel filter
Injector seals


What could it be??

ECU??
O2 sensor??
Fuel injectors??

HELP!!!