I have no reverse when it rains or sometimes after a wash. This has been going on for awhile but hasn't been too big of issue as it seldom rains in the desert. Until it dries out when going forward the speedometer reads at a higher MPH than I am actually going. This past weekend it happened again when leaving on a boating trip. I washed the boat the evening before attached to the truck. In the morning I drove into town to drop the wife's truck off at the shop for some work with no problem. I left the shop and the speedometer jumps to around 70 when I am doing about 40MPH. it clears up as it has in the past and I jump on the freeway. I drive about 20 miles at 65-70 MPH with no problem then the speedometer suddenly pegs and the truck shuts down. I restart and turn around at the next off ramp and head back to town with no problem. I get back to town and it does it again. I pull off the freeway and drive to the shop at about 40 with the speedometer at around 70 -80. Since my normal mechanic won't touch a Powerstroke I take it to the local Ford dealer. They say they cannot find the problem as there is no code and the truck runs fine after sitting the weekend
. Has anyone else had this issue and know the solution? Also the RPMs are normal for the speed driven.