could use a little advice

[90 cheyenne]

I have a 90 K1500 cheyenne 350. Awesome truck, love it, simple but functional, I have a miss that i cant seem to get rid of though. Really noticeable at idle. I was doing a minimum air rate check on it and when I unplugged the iac motor and started the engine, it was smooth as glass!?! Even in gear,smooth, but plug the iac back in and start the engine, and it misses again. The iac goes through all its steps as far as warmup, so I know its not that. If anybody has any suggestions, they would be greatly appreciated!!!!!