hey, welcome to the forums.....first we need to know what year is your car and what mods u have, is it an auto or 5 speed, etc.....there have been a lot of posts on weird idles so u might want to do a search....one of the main things it could be is disconnected vaccum lines underhood, loose airfilter, IAC or PCV valve....u might want to check all these things first....also, how many miles does your stang have....have u tried a fuel system cleaner or some sort of treatment u can put in the tank...first thing i would do is check all vaccum connections and check the filter.....then check the IAC and PCV valves....a lot of people have had to replace these parts to stop the problem.....let us know