just to clarify,
you had a adm sponntah, with a t50 box and a set of wheels on it.
and you changed only the gearbox and nothing else (in the drivetrain)?, and your getting funny readings from your speado cable?
you haven't changed your diff ratio at all? or you haven't put different sized tyres on the back?
the fact that the speedo is jumping around at low speeds, suggests to me that the speedo cable isnt in the best shape, but a speedo cable is just that. its a cable that spins, i would have thought it either spun or it didn't (ie a broken cable, in which case the speedo should do nothing at all).
unless the end that goes into your dash is slightly bent or something? and gets caught at low speeds?
one thing you could try, is putting the speedo gearing from your old gearbox (if you still have it) into the new gearbox?
im of the understanding you can interchange the little connection that goes into the gearbox. though i myself have not done it.
jsut an extra thought, dont jap 86's come with 14" wheels? and adm come with 13? could that account for the difference in speed? (+/-10km/h or so). in which case changing the cable gearing as suggested above should do the trick...