I had the same problem today. I had just got back from a 300 mile round trip pulling my 5th wheel (truck ran great). Dropped my 5th wheel off at my storage, and when I got on the road I noticed my speedometer was not working. I just blew it off but when I started up a descent grade I had no boost and very little acceleration so I started to pull over and then I heard a loud click and all of a sudden I had full power and my speedometer was working again. I drove another 30 miles after that with out any problems. It makes me think it is an electrical problem. My whole EGR system is gone so that is out the window for me.