transit speedo not working?

Hi my speedo is bouncing around on my 54 plate transit minibus. It is a PSV vehicle so a separate tachograph is also fitted to the vehicle and both the speedo and tacho are both bouncing around and not reading acurately. I have changed the VSS, the speedo head unit and the speedo transponder but no joy.
The first thing I noticed before the problem started was that the second digit on the milage meter was showing a dash on occasions instead of a 3.
Secondly it all started happening soon after we had a new reconditioned gear box fitted but had it back to the gear box specialist and they are addiment its nothing in the gearbox.
Can anyone help as I've been trying to resolve this for over 6 months and its been into 4 different garages on over 10 occasions.
Many Thanks