Join Date: Aug 2006
Location: Halesowen, West Mids
I had this identical problem on my old 156 and found out what the problem was from this forum, after months of checking the speedo, ABS etc.
This will almost certainly be down to a bad earth. Either one of the battery leads is loose or broken inside the plastic, or theres a problem with the engine earth.
Once I had checked my earth leads I dismantled the leads in the black box that sits ontop of the battery. Cleaned them up, greased them and ensured eberythng was tightened up. It solved it foe me.
Check them out, it cold be as simple as a loose battery lead.