Cliff,
I had trouble with mine right after the truck was back on the road after its restoration. After some trial and error, it turned out to be a bad reducer gear that the speedo shop put in to compensate for the new tires. Replaced that and it works great now! Just a thought.
I don't know if you even have one on your trans, but if you do, it is easy to check. Remove it and reconnect the spedo cable. If it works, the reducer is the problem.
Good luck!
