Sounds like a problem with the magnetic-reed switch 'sensor' bolted to the transaxle. Either a dirty/loose connection at the two-wire connection to the body wiring harness, or a bad, sticking reed that has to be warmed up and vibrated a bit before it 'wakes up' and starts working.
As stated above ... if the odometer works while speedo doesn't it isn't a sensor problem!
If you have an oscilloscope and jack stands, put the car up on stands, and monitor the two sensor leads while still attached to the body wiring harness. With the engine in 1st or 2nd gear and the rear wheels turning ... you should see a 6-Volt peak-to-peak pulse train ... eight pulses per ring gear rotation.
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Warren Hall, Jr.
1973 911S Targa ... 'Annie'
1968 340S Barracuda ... 'Rolling Thunder'
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