I have a 1996 Taurus LX with the Duratec, about 115000 miles. Car runs fine, and when in gear, idles at about 650rpm. When the car is warm, and I put it in park, the rpm's drop to 450-500rpm, and the engine stumbles a bit, but never stalls out. Sometimes it does this in reverse, too. The PCV valve and hose was replaced last September, and the IAC last August. I can't find any vacuum leaks, and there are no codes. Anyone have a similar problem or know how to fix it? Thanks.