Jumpy speedo needle

Hi john My 85 SWB speedo did this all the time. The cable was fine however, so I think what happens is related to the moisture you often get in G's due to failed spindles etc, which gets into the speedo and affects the way the friction plate - my understanding is the cable transfers rotary into steady speed via a magnetic 'clutch' - operates at low speeds. As the cable turns faster, the momentum overcomes this effect, and happy days :-) Now I said did, because mine went onto stop reading until the dash was tapped, so I sent it to Speedy Cables and had it reconditioned and it worked fine, no jumping etc, until it stood for 4 months having body work done. Now it's usually fine, but very occasionally needs a tap to get it going. Hope it helps. All the above is a combination of various info from other much more informed members and my own conjecture Ta Simon