On my 110 Td5 the speedo sometimes flicks up to 50mph just as I'm about to fire up the engine. Done it a few times over the last few years - not sure why?!!!!
There something about the output shaft for the speedo shifting or even following the revs. Think speedo capput. Maybe not the cable. Way to check is remove the speedo from dash and connect a drill up
you could try fitting a suppresor after the alternator as you may be getting voltage spikes which can cause some crazy gremlins in your elastitrickery system and will cost you all of a fiver.