There are rare cases where the speedo drive gear slips. Causing lack of speedo function (intermittent).
If your speedo always works, then you need to monitor the speed signal at the ECM, the signal it receives from the Instrument Cluster.
You can do this with a Digital Volt meter connected to the correct wire at the ECM harness plug, or via a good scan tool such as VCDS if your OBDII reader can not display live engine sensor input data.
Monitor the voltage while the car is driven or jack-up & support the car and then let it freewheel in gear while up in the air.
Wiggle the wire / harness all the way to the Instrument cluster to see if there are any sensitive spots to vibration / movement.
Might not be easy to track down. If you are lucky, it will be in the raintray area where the ECM is.
Friday, October 19th, 2012 AT 4:39 PM