I had the same problem in my old 200B wagon several years ago, having similar symptoms as you with regards to it occasionally working again. It ended up being the pinion drive. Look carefully at the gear. Mine looked fine at first site, but when examined closely it was found that the middle of the gear had stripped out just enough so that it would no longer work. I replaced the pinion drive and problem solved, I had a speedo again!
_________________green200b - aka Alistair
'80 200B GX - the green200b
'81 RX-7 - 12A extend port, 4.11 Torsen LSD - daily
'74 RX-2 - 13B bridgeport, 4.44 LSD - cruiser
'82 Hilux - 350 Chev, 4.3 Detroit Locker, 33s - 4WD
Green200B has forgotten more about 200Bs than we will ever know.