i have a 96 zetec(4 cyl.) mtx too with over 120k and mine does the same thing. it could be your idle air control valve (IAC) needing to be cleaned or replaced, expensive from ford though. first however you should clean your mass air flow sensor (MAF). its the first sensor after your air filter in your intake tract. unplug it and unscrew the worm gear clamps to release the entire metal cylindrical section. once you get it out if you look inside you can see the two thin wire sensors extending into the smaller interior metal sampling tube. spray them with residue free electrical connection cleaner until they no longer look black and dirty. should have a thin metal wire look to them. reinstall, plug it in and crank her up and see if that helps. dont forget to disconnect you negative battery cable before you do this and reconnect it when your done. this is always good practice when working on electrical portions of your car and will allow your computer to reset and adjust to the now clean maf.
