I own a 00 Windstar and had the same problem you
decribed. here is what I did to solve the problem.
I took the IAC valve off (it's located on the top of the intake on the drivers side). IT has 2 5/16" bolts holding on with a plug in on the driver side end of the unit. I took WD40 and sprayed it in there to get the carbon build up out of it and my van runs fine. Just a suggestion, hope it works for you