I've had this problem for as long as I've had the car, well, after the original REMAN ECM crapped out... No idle speed control whatsoever. If you're like me, you have replaced the IAC, the ECM, and maybe even the pigtail wiring for the IAC. I ended up with some idle control... Only AFTER I would rev the engine after starting it. ONLY THEN would the ECM set the engine speed to 1300 RPM or thereabouts.
Now, I have a 3.1, with the later-style IAC and throttle body, used on the 88+ 2.8 and 3.1, meaning that my idle air supply comes around the throttle plate within the throttle body itself, not through this nylon pipe thing that runs from a nipple on the TB to the nipple on the intake manifold (not the lower plenum) that is most commonly used for the PCV hose... I deleted that with a 3.1 top end (to go with my engine) because my nylon pipe thing broke (hence 4 years of no idle control) and my upper plenum decided to break at one of the TB mounting ears and the welder screwed it up so that the TB mount still leaked air into the engine (read as: BAD thing). This TB has a non-adjustable TPS.
After cleaning my IAC, AGAIN, I decided, "well, let's just see one thing. It seems as though the ECM doesn't even realize that the throttle is closed for some reason. How's about I do something different and check my TPS voltage?" So, I did, and my TPS, which is supposed to be sitting at .55VDC for closed throttle, was sitting at .76VDC!!
Keep in mind that I said before that my TPS is NON-adjustable. So, how did I fix that? I took my Dremel clone (RTX) with a grinding bit and made the mounting holes into slots. After reinstalling the TPS, I adjusted it to .55VDC, and I now have idle control.

To make a long story short, if you are at your wits end because you have done practically everything under the sun to try to get your idle speed harnessed (also sets a code 25 if it goes on for too long), try checking your TPS voltage and adjusting it using any means necessary to get it back in check, even if it means replacing your TPS.