I skimmed through looking to see if anyone posted this but if so then its my bad and im sorry for that.

start the car get out of the car and walk around to the rear tail pipe. and just for the heck of it " caution dont get crazy and grab it with your hands" check to see if any exhaust is comming from the pipe.

if so the look under the car to see if the y pipe is bent or crushed, if no exhaust comming from tail pipe then look at the cat is it getting very hot or red?

hope you get it fixed.

I'm not sure how much this will help, but my dad was experiencing similar problems with his 87 gmc pickup. it has a 454 tbi setup. when the engine was running, the idle would surge up and down, and sometimes would die at stop signs or when pulling away from them. we tried replacing the tpi, map, and o2 sensors, computer, and even had the throttlebody completely rebuilt to no avail. he was getting a lean exhaust code. recently he unhooked the line going from the air pump to the air injection tubes on the exhaust manifolds and it solved his problem. evidently somehow it was tricking the computer into thinking the exhaust was lean giving the engine more fuel which would cause the surging. im not sure if your car has these tubes but its something to look into.

good diagnoses

kinda had that problem....didnt get to read all the thread so dont bash...it was my fuel pump!!! how gay

I would recommend that you remove and clean your IAC motor. These things get really dirty and will act up. It screws in under your throttle body.