I'd have to say you should go right to the TPS and base idle reset with this one. If that's not it, go to a junkyard for an OEM IAC to rule it out as the source. Look for one that looks decent, inside an out, to minimize the chance that you end up with a bad one. Clean it out with some carb cleaner and Q tips before you install it.
'89 GT, 5-speed, (because it's boring when the car does it FOR you), freshly rebuilt engine and tranny, GT-40 heads, Performer upper and lower intake, 130 amp alternator, K&N, alternator upgrade with Taurus fan, DCC fan control, subframes, Cobra R 17X9's.