I have an intermittent fault with the speedo on my Mk2 Mondeo.

When driving the speedo will just stop working. When it happens the fuel gauge will drop as well, not to empty but to the poisition it would take if the engine were stwitched off.

After a while the speedo will start working again, when it does the yellow petrol light (bottom row of lights righthand side) comes on for a couple of seconds.

Is this an electrical fault in the dash or a sensor problem in the gearbox?

Many thanks for any advice.