I've got a 91 GTA 5.7L TPI. Its all stock except for a new cat back and K&N filter. About a year ago the idle started to become a little rough but it was only noticeable when the engine reached operating temperature. Well, as time went on the idle became rougher and rougher but still only noticeable when the engine reached operating temperature. Eventually it became so rough that the engine began to surge. When I start the car the engine idles PERFECTLY but after half a minute of warming up, the idle starts to become rough then eventually begins to surge once it reaches around 100 degrees. The surging is about 100-200 rpm variation in a steady rhythm. When I'm park or neutral I can rev the engine all the way up to 3500 rpm before the surging disappears. When the car is in gear, it tends to torque the wheels when I'm at rest and there's a tendency to stall sometimes. But when the engine is under enough load it drives ok but soon as I let off the throttle the engine jerks the car. Its really a handful to drive now and I've been battling this for over a year now. The service engine light has never come on. I replaced the O2 sensor, did a tune up, checked for proper EGR operation, vacuum leaks, proper IAC operation. I'm not sure if you can do this, I disconnected one sensor per run to see if I could isolate the problem. I tried the MAP, MAT, CTS, and knock but no luck. I took it into the dealer and the mechanics couldn't figure it out either. They checked the ECM, did an injector balance test, and checked the ignition timing. They said the manifold might be cracked but I'm not so sure because it idles perfectly when its cold. Anyone ever had this or can help me out at all?