IIRC, with the tacho, its actually a little servo inside that is controlled via the revolution signals from the crank angle sensor. If its bouncing around, then i'd say you need to replace the servo, which you cant. You have to replace the entire instrument cluster.
I could be wrong though. I know for sure the z32 instrument cluster is all electronic which is a bitch because my 85 zed has a speedo cable that connects to the gearbox. The newer gearbox however doesn't have a hole where the cable can go because its designed to be used with an electronically controlled dash.
-Mike