Car hesitating, don't know why

Ok, so my 97 LTI is hesitating under acceleration. When accelerating from a stop with moderate throttle, the car will hesitate and want to stall.

Last year, we had new headers installed, but used the stock 02 sensors. New plugs and wires were used. Installed a tune by PCM4Less for the headers and new rear gears. We installed a new Walbro fuel pump and filter while we were trying to diagnose the hesitation, the oem was running a little weak. New TPS sensor was installed. Optispark was replaced by previous owner [bought car with 29,999 miles on it in 06, car currently has under 40k].

Ok, on the PCM4less tune, the car hesitated on his tune on the ecm he shipped us, but the car ran fine on the oem tune. So we drove to his place in PA, and he discovered we had a bad ecm [the one he sent us was bad], so he flashed the oem ecm with his tune. But the hesitation came back.

Could it be that the 02's are going bad due to the headers? I know you are supposed to replace the 02's after the header coating kills them, but I have not received a CEL.

Any ideas?

How long after the pcmforless PCM replacement did it run OK, or did it start hesitating right after he flashed your original PCM?

Have you rechecked the fuel pressure?

IF the O2 sensors were contaminated by the header coating, it probably would have shown up very early after the header install. It would affect the system whether you were running the stock tune of the pcmforless tune. And, when they get contaminated, it may be just enough to throw them "off", but not enough to cause a code. They may be switching at the correct rate, but the voltage is "off".

Have you checked for something very basic, like a torn inlet elbow?

It ran about a month ok after the tune was flashed, then we put it in storage for the winter. Fuel pressure was good after the new pump was installed. I'll check the elbow when I get a chance.

try replaceing the fuel filter, its possible to have the right pressure with out have the proper amount of fuel.

check for burnt wires.

:(my 95 did the same and it was a coil wire arcing and when replaced- problem gone,,,at night with it running check for jumping sparks. In the day you can use a water spray bottlle to loocated the wires that leak. :)