i have had the same problem ever since I had my car, I've done everything known to mustang kind to fix it. when i did my H/C/I I replaced my TPS(adjusted properly) new IAC, New Throttle body,new O2 sensors, and even the bypass plate that is recommended by Ford to stop the idle surge. the only option I have decided on now is that maybe my coolant sensor is crap and is telling the computer that it is really wamer than it is, so maybe that's another option to consider, but first I would go with cleaning out the IAC, cleaning the TB, readjusting the TPS, make sure the main wiring harness connectors or connecting properly. I don't know if anybody has it or maybe you can search google but there is a site dedicated entirely to mustang idle problems.goodluck with your problem.