JMH

Before you spend any money, try this...

A poor earth connection to the instrument cluster casing is a common cause of needles - fuel and temp - jumping randomly on these 123 series. If a sharp thump on top of the cluster causes the needle to jump back then the earth is certainly your problem.

The fix is to pull out the cluster and connect a separate wire from the cluster casing to a good earth point somewhere behind the dash. Two out of my six 123's had this problem.

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Beagle