P0505 Idle Air Control Circuit code

I have a 1998 Plymouth Voyager (base model) with the 2.4 Liter 4 cyinder DOHC engine.
Looking for help on P0505 code, Idle Air Control Circuit.
The check engine light is on; I clear the code and the light goes out. The car can idle for quite a while without the check engine light coming back, even when I rev the engine. But as soon as I put the car in gear and start driving, the check engine light is back within a minute.
I have done everything I can find on the internet on various OBD II troubleshooting websites. I have replaced the idle air control valve/motor and the throttle position sensor. I have replaced the air filter and thoroughly cleaned the air intake and the throttle body; I have cleaned the electronic connections.
After all of this, the engine does run smoother, but the check engine light keeps coming back. The only thing I haven't done (beyond my abilities I fear) is replace the wiring harnesses.
Anybody out there have any other ideas?
Thanks.
Art