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There are quite a few things that could cause it, unfortunately. A vacuum leak is normally the culprit, from what I've found. Although, a bad leak normally caused a high idle. Anyway, check all vacuum lines and the seal around your intake... including the rear lower intake where your pcv is located. You can check by spraying a little starting fluid or brake cleaner around the engine bay during idle.
You IAC valve (idle air control) is another common culprit. Simply remove it from your throttle body and clean it as well as the two openings where it connects. I had to replace one recently and the new one was also bad.
You can also Google "base idle reset" and go through the process of setting the idle, but you need to make sure all other issues are taken care of before doing that.
Ignition timing, dirty mass airflow sensor, bad O2 sensor, worn ignition components, and wiring issues can all cause problems with surging idle.
If you do a search for "surging idle checklist" there is a good one floating around the mustang forums. I've used it before and it was very helpful.