From NicoClub:
Most of the speedos and tachos in that era used balanced air-coils and not stepper motors, and what you're describing sounds like a low side coil with a crappy ground. It won't be a problem with the VSS because the car would be throwing codes, so whatever the problem is, it's almost certainly internal to the cluster. If you're handy, pop the cluster apart and look for degraded solder joints on the mainboard. Refreshing the solder connections will often fix up that sort of issue.