Vacuum sensor and electic fuel regulating solenoid maze of complicated gadgets is my guess. Did you see Greg Bambo's and Art Dalton's excellent discussion on missing when cold and the more primitive 6 cyl IP vacuum valve?

Sorry I can be of no more help, but somebody more knowledgable with the 4.5 should have the answer. A list somewhere of sensors and solenoids that oughta be picked from junkyard cars would be helpful, rare and endangered parts, that can be swapped and traded here on the forum.

The problem sounds related to fuel/air ratio at idle to me.