I would agree with checking idle air control motor. Take it off, clean it out with carb cleaner and put it back in. Also, Fordspeed sells a plate that goes between IAC motor and throttle body. That fixed the problem on my friends' car.
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John
1992 Mustang LX 5.0-Powerdyne BD-11A, 3.73's, FRPP clutch, chrome Ponies, Mac headers, Aeromotive regulator, MSD 6A, MSD BTM, FRPP 42# injectors, Pro-Flow Air Optimizer, Trick Flow Track Heat intake, Edelbrock 75MM TB, AFM Power Pipe