Mine had the same problem and removing it and spraying the gears with electrical contact cleaner flushed out a bunch of black gunk. Then the gears didn't get stuck any longer and it's worked fine for a couple years now.
There was a writeup on here by a Swiss guy in the FAQ > AutoFAQ section but I see the link is not working anymore. :-( You might try Google and search "z32 odometer fix", as someone else may have copied the info. If you can't find it email me and I'll see if I can dig out any printouts I may have used if I still have them. Then I could scan them and email them to you.