2000 2.4 - rough idle

2000 Grand Am, 2.4 DOHC, 205k miles.
This is an update to my 11-29 post because I think that the problem has become clearer after making some repair attempts. Earlier I reported that the car would stall at stop lights after it warmed up. Since then I have done the following:
Installed new plugs, installed a new air filter, cleaned the idle air control valve, removed and cleaned the throttle body and replaced the mounting gasket, cleaned the inside of the air/oil separator hose (comes from the timing chain cover), cleaned the hose that comes from the bottom of the resonator, changed the oil, added head gasket sealer (Bars-Leak), had the ignition control module checked at Autozone (good) and replaced the coils.
After all of that, it usually does not stall at lights and my rpms stay constant at 600 when stopped, but the idle is so rough that it can cause a stall infrequently. So I have come to the conclusion that the problem is a rough idle. I used to have a weak rough idle and now I have a strong rough idle. Also, the car idles very smoothly at high idle when cold.
Still on my list: Have the fuel filter changed.
I will appreciate any other suggestions.
Thanks.

I'ld still suspect a bad IAC valve. Could be damaged.

See how it is after you change your f filter. its cheaper than an IAC and something that should be done with routine maintanance anyway.

The only task of the IAC is to deliver air to the intake in whatever volume is required to maintain the target idle RPM. The air is distributed equally to all cylinders via the plenum. It cannot deliver more or less air to any individual cylinder, therefore cannot affect how an individual cylinder fires.

If the engine is maintaining its target curb idle RPM of 600, the IAC is working. If the idle is rough it is due to one or more cylinders firing differently, this is not the fault of the IAC, but more likely incorrect fuel delivery to one cylinder, weak compression in one cylinder, or weak ignition in one cylinder.