Peugeot 406 1.9TD 1996 speedo flickering?

Ok so when driving along, my speedo sometimes is fine, sometimes doesnt move or will jump up stick at a random speed. When i hit the dashboard sometimes it settles down to the speed which im travelling at, sometimes will drop to 0 (when obviously im still moving) or will stay were it is until hit a few times.
Im thinking possibly a loose connection behind the dashboard but have no idea where to start!
Any ideas guys? Im at a loose end here, and although I have a Haynes manual, there is NOTHING in there on this hehe. Also, as sort of a novice, if any suggestions please provide instructions as I dont mind doing it myself.

Regards in advance,

Jack