I believe the problem stems from the plastic on the back if the instrument
cluster getting old and brittle. (I've seen this on both an '85 and my '90.)
Then is breaks and you find the speedometer housing is loose in the back of
the cluster. Mine is held together with lots of epoxy.
Be very careful of the slide-on wiring connector on the back of the housing.
The copper traces on it can come loose and then you are destined for chasing
bad connections through the connector. I finally had to abandon my factory
setup and solder wires to the connector points on the various components on
the instrument cluster. These are then wired through an after-market male
female connector. You need 12 connections on my Vanagon & I used two six
conductor trailer wiring harness. I got them for $2 each at Trak Auto. It was
a long evening of soldering, but well worth the effort