2003 Vauxhall Astra iratic speedometer

when the car has been switched off for even a short period(1minute) the speedometer will not rise immediatly when the car is started and driven. It seems to kick into life at almost 30mph or at least that is the speed it shows apon moveing again. When it is active you can slow down and even stop and as long as you dont switch off the ignition it will rise immediately. On occasions the needle will fall and rise again whilst travelling along.