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Sounds to me like the speedo cable is binding somewhere... probably somewhat close to teh speedometer. Or it could be the speedometer head itself.
Basically, causing the top of the cable to stop, while the rest of it twists up tight, then releases causing it to jump to 100mph+. Then it binds again and reads zero as the cable again winds up.
I'd start by first checking that the cable is securely on the speedometer, and that there are no harsh bends anywhere. Then try sending some oil down the cable sheath itself and see if that helps.
You could also pull the speedo cable and watch it...see if it moves smoothly when you drive, or if it stop/start/stop/start etc...
And...where did you find your long cable? I just put a OD in my '70 145, and went to get the long cable and they couldn't find a part number listed... Do you know how long exactly your new one is?
-Matt
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