Anyone have any input into this?
I've been living with my odd tach behavior, as it's not really a
problem, but I would like to fix it. This is an '85 cluster in an '86
wolfsburg westy.

My exact symptoms:
-starting when engine is cold, tach reads normally at ~850 rpm
-as van comes up to operating temp, the tach needle will slowly
read higher and higher at idle. during this time, engaging various
electrical switches (blinkers, stepping on the brakes, turning on
parking lights, etc.) will all effect the tach needle slightly.
-once van is up to operating temp, needle reads right around 4000rpm at idle.
-once engine is revved above 2k, needle reads accurately.

It will stay like this indefinitely once the engine is hot.

The multi-pin seems seated properly in the connector, the foil
looks good, and all wiring looks fine. I pulled out the 3-pin connector
that's attached to the foil on the back of the tach and sprayed some
contact cleaner on it just in case that would help, but no change...

I still am inclined to think this is a ground problem somehow
related to the coolant temp sensor, since the tach reading changes in conjunction with the coolant
temp, and those other things that effect the reading.
Should I run a new ground wire and splice it into the brown wire in the multi-pin?

I'm sort of at a loss...
although I've thought about just buying an '86 or newer cluster and
putting in my current speedo/odo, but the rest of the instruments work
perfectly.

Any suggestions are greatly appreciated.