could be the worm drive on the end of the speedo cable in the gearbox or the spring on the needle,
follow cable to where it enters the gearbox and you will find a little plate held on by a bolt, undo bolt and carefully pull out cable[ do this very gently as you dont want worm drive to catch on casing and fall into box!!!!!!! ]
inspect the gear for missing teeth/damage etc and replace if necessary.
if thats ok then its likely to be the speedo itself, replace speedo as the needle spring or gears in speedo are fubar
