Idling too low Problem

I have a 99 Dodge Intrepid. The car usually runs smooth but when you drive it for a while and it heats up when you stop at a traffic light, stop sign, etc., the idle will drop low (around 500 rpms). On occasions it will drop too low and actually stall on me. It only happens when the car is warm. Driving it longer in stop and go traffic makes the idling problem worse as it will fluctuate up and down between 500 and 200 rpms. My mechanic has changed the idle control sensor but that hasn't solved the problem. There are no codes being displayed either. Any suggetions?

need to have your mechanic do a signal check on the signal recieve line to i.a.c. supposing you do have a good iac.motor..and the bore is cleaned,throttle body cleaned,a computor or signal line issue may be to blame.if proper signal is present,tap on the iac carefully see if idle changes.the pcm controls all the signal.give a try and re-post.