Wise move to disconnect the cable.
What has happened is your speedometer is completely dried out. This happens to all old cable driven magnetic speedometers. The cure is to remove the speedometer & send it to a repair shop. These speedometers are not field serviceable and at this age it's long overdue for an overhaul. Cleaning, inspection, replacement of worn parts. Re-magnetize and calibrate. This should be done around once per every 100,000 miles or 25 to 30 years whichever comes first. There are 2 oil-lite bushings inside of these speedometers. One big one and one tiny one. Over time (as well as miles) those 2 bushings dry out and the magnet shaft trying to turn inside of the bigger bushing without any lubrication is what is causing your pointer to "jump." Running one of these old speedometers dry; the magnet shaft gets all torn up by lack of lubrication, often to the point where it's ruined.
I have taken apart 6 old speedometers in the past month and; all of them the magnet shafts are ground up. The steel touch surfaces actually turn blue and purple on account of the heat (friction).
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Peter Serio
Owner, Precision Pontiac
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