Remove the IAC sensor on the side of the throttle body and clean with carb cleaner. You will need a gasket. Motorcraft will want $20 and they are not at the parts stores. You can make one or pick one up at sites like Latemodel Restoration Supply - Mustang Parts, Chevy Truck Parts & Dodge Challenger Parts
You may also want to inspect the inside of the throttle body and clean out any residue. Do not use carb cleaner inside the throttle body as it may remove a coating that is supposed to prevent build up of gunk.
These are the top 2 most likely sources.