Considering that 914's average approx. 23K+ years-old, chances are your speedo cable is:
- Dry as paper i.e. no lube, causing it to "hang" within the conduit.
- Has a kink in it somewhere; more-than-likely near the tranny end.
- Your speedo needs attention because of 914 age indicated above.
Most of the time, it's the cable. It has to be maintained relatively straight all the way to the tranny gear drive, without a lot of "bends".
Mine does the hypnotic bouncing from rest to around 25 MPH, but I can live with that; I'll get around to changing the cable.
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Rouser!