Lots of people forget to clean their MAF, ACT sensors and as well forget to clean their IAC. You should also check your setting on your TPS. Doing these 4 things can cure lots of common 'stumbling' and cure many idle problems. These are easy steps to do and should be done once a year at least I would say. When you are doing this once you are finished reset your computer by taking the negative terminal off your battery for at least 30 minutes.
FOR MORE HELP VIEW THIS LINK:
http://www.muscularmustangs.com/maintain.php
FOR MORE HELP VIEW THIS LINK:
http://www.muscularmustangs.com/maintain.php