I've spent a lot of time chasing this one. Speedo works very intermittently (5% of the time on my '84 5 speed 5000S wagon. I have replaced:

Speedometer cable. (professionally modified automatic version to match my manual one which is NLA.)

Speedo Cable Drive gear in transmission.

Swapped out to my spare instrument cluster, but in driving the car around without the instrument cluster in place (to troubleshoot only, mind you) I can see that it's the cable that is not moving. I've also put some pressure on the cable to ensure it's seated down below with no better results.

In my mind there is only 1 remaining culprit, which is that whatever gear inside the transmission which drives the speedo cable drive gear must be bad.
Any ideas? I can't find any exploded parts diagram that identifies this piece.
I'm not keen to rip into a transaxle but would like to have a functional speedo / odo.