I had nothing but problems with my idle air control valve. I replaced it more than once hoping to correct the same kinds of issues you are describing. I also tested, adjusted, and replaced all sorts of things (Throttle positing sensor, MAF thingy, etc.) and never could get the IAC valve to function properly. Finally, I made it close all the way and then I unplugged it. Car has run like a champ ever since. I'm really not even sure what the thing does. I think it helps your car to warm up faster by making it idle higher when the engine is cold.

'91 Trooper
3.4 V6, K&N, 33x12.5 Trxus MT's, Aisin hubs, 3" body lift, Rear ARB Naches Trail