it is electronic speedo try replacing the vehicle speed sensor in gearbox and also common fault the wiring from the vss plug corrodes due to all the heat from surrounding water pipes check the 3 wires in the plug you should have a 12v live an earth and the last one is pulse wire which goes to the clocks via the throttle position sensor is it misfiring at low revs when the speedo stops?
wiseguy82, November 2008