96 B4000 speedometer question

second question for the day ... what's wrong with my speedometer? most days when the truck is cold the speedometer will work starting out, usually for anywhere from a couple to maybe 10 miles, then it stops working. i've checked the obvious stuff like fuses, etc. my repair manual is pretty sad in this area, but it looks like i've got an electronic speedometer, no physical cable exists that i can see. any assistance on where to begin looking is appreciated.