Mine does that too. I just live without a speedo but I'm sure it's the speedo head at fault. It would wind the needle all of the way around the face of the speedometer just before it would kill the cable. I know there are speedo repair shops out there, I would contact some and see if there's any common problems to the A.D. truck speedo's and get some quotes to get it repaired.
Good luck with it,
Scott
P.S. got pictures? and welcome to the 'Bolt
