1989 Firebird Formula 305 TPI auto trans. After driving for a while when I come to a stop light or sign an sit my cat start to drop rpms from 700 to 500 sometime more than it will kick to 1000 and drop to 200 or less and die or kick back up to 1000. Also throws a lean exhaust code for a couple minutes then goes off and this happens randomly, I have replaced the TPS sensor, pcv valve, iac valve, fuel filter, spark plugs, O2 sensor, all 8 fuel injectors and I have cleaned my upper intake and throttle body and relearned the idle air control. Also checked for vacuum leaks but found none, I was told to check for vacuum leaks again, do head gaskets, or replace my crankshaft position sensor or distributor. Don't know what to do next please help!!

 

How are you checking for vac. leaks??

 

I have sprayed the hoses with carb cleaner, soap and water, and there is no noticeable hissing sound.

 

Good Idea,i was going to suggest Starter Fluid.

 

Could be egr related

 

Is the "Service Engine" light coming on??if so what is the Code.

 

Maybe check the fuel psi too.

 

my car did that many years ago, turned out it was the mass air flow sensor. once i replaced it the hunting went away.
on a side note, once your car is at idle disconnect the IAC then drive it to see if it still hunts. maybe it would help to see what component is causing the hunting, it is fuel delivery or air.
when you cleaned the throttle body dont forget to remove the IAC and clean the heck out of its cavity.