have you tried cleaning the throttle body and idle control valve? pretty easy job and can't hurt, one of those things that may not have been done in 20 years (pretty sure mine wasn't when I did it)...
https://www.youtube.com/watch?v=slPzoplzWT0
I couldn't get the throttle cable clip out of mine, so instead of mangling it I just rotated the body upwards and cleaned it in place.