I think the 700 series speedometers are prone to have weak solder joints on the PC board. I had an erratic speedometer for years and eventually sent it off to Overseas Speedometer (in Texas I think). They did whatever they had to do regarding re-soldering the board and it has worked properly ever since.
Had it done about 5 years ago or so. Can't remember the exact number, but it may have been around $100. I wanted to keep the original cluster instead of swapping in another that may have suffered the same fate.
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'86 745T, '72 144E, '70 145S, '68 220S, '60 544
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