It might be the plastic gear on the VSS. Or the cluster is possessed.
I have a similar prob. When I start moving my speedo will go straight to 110mph and stay there regardless or speed changes. Sometimes as I slow down the needle will all the way around to the under side of the pin then back to 110 shen I speed back up.
If I tap on the cluster as I drive sometimes it will read correctly and sometimes not. So I'm sure my prob is in the cluster. But where.
the last time I has the cluster out I added a "paper shim" under the PCB on the back of the cluster so the 2 connectors would make better contact. It seems to have helped but I think I need another to cure it completely.