My 99 XLT 4x4 was doing the same thing. I have 160,000 miles on it and the problem just started. I pulled the IAC and cleaned it. It was pretty dirty. I then checked the unit and the system as outline in the Hanes manual. Everything checked out fine so I reinstalled it.
The whole process took about 15 - 20 min max.
So far so good.
Would probably be a good place to start.
Mark