sometimes my needle starts wobbling up and down either side of the current speed I am doing, most of the time it it perfect and usually does it when I give the car some stick quite often on the motorway and then when I slow down it seems to start working proper again.

 

diff bearings, speedo cable

 

change the speedo cable and if that doesn't solve it start looking else where, speedo drive, speedo head, like pugo says possible diff bearings but you'll normally find a oil leak to go with that.

 

cheers all