Why has my Ford Focus' speedometer has developed an intermittent fault?

I have a Ford Focus estate with a 1.8 Zetec engine, registered in 2001. It has now travelled about 75,000 miles with no problems. Recently a fault has developed: the speedo and milometer stopped working intermittently. All the other dials remain working, so I have to judge my speed from the rev counter. The speedo stops working for 5-10 minutes then starts up again of its own accord, there is no pattern to this failure. If I am on the motorway or driving along I can feel a slight loss of power when this happens, and a slight increase when it recovers. The dangerous bit is if I am crawling along in traffic, or changing gear when the meter is dead as the engine cuts out and I lose my power assisted brakes and steering. I have looked on the Internet and this fault in various forms seems common on older Fords, and a number of models but no single cure is given.

Asked on 3 March 2012 by LP, London N6

Answered by Honest John
Hopefully, quite a simple fix. It's usually caused by failure of the oil gasket to the speed sensor in the final drive. The repair costs about £100. The reason for the other problems is can-bus electrics. When one component on the power supply fails, this can affect others on the same power supply.
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