idol problem

Van starts a little hard and if you let it sit and idol it idols low and high and just keeps jumping all over the place. It cuts out a second and then comes back. If you are driving it seams fine until you are stopped. Machanic tested all computer and fuel and everything checks out fine. He is stumped too!!! I just can't figure it out!!! Any ideas??
This is a 1994 E350 5.8 ltr fuel injected van.