I've got a '92 Trans AM L98 all stock with the following symptoms:
I start up the car cold and it runs fine. Once it warms up the idle is rough and feels like its chugging. It only does this at idle, when driving at speed the car runs fine and there is no loss of power. RPMs remain constant and do not surge. Car has not set any codes.

Spark plugs are about 3 yrs old however I've put less than 5k miles on it since then. Also - plug wires, cap and rotor all replaced a few months ago with good aftermarket stuff (I double checked all the connections last weekend).

Now I think I stumbled onto something last weekend: I have a K&N filter clamped right onto the throttle body (I've heard this nicknamed a "hot-air intake"). Anyone whose done this knows when you start up the car cold you really hear the air getting sucked through at idle and when you press the gas its even more noticeable. However when the car warms up the sound basically goes away. What I noticed, is the sound does not go away it continued for the whole 20+min car ride. Its only done this once (I only drive it on weekends) so I'll check more tomorrow.

If this is the case I'm thinking the computer is stuck in 'warm-up' mode? Does that mean I need to replace it? Or could there be something else? I realize there could be many things causing a rough idle but if someone could give me an idea where to start trouble shooting I'd appreciate it!

 

OK - scratch that last part. I drove it this weekend - car still has rough idle but I didn't hear that intake noise.

Anybody have any suggestions?? I checked the timing and its fine -- I guess I'll just start swapping out sensors and hope that might cure it.