There is a thingie called a voltage stabilizer on the rear of the
insturment cluster. I think it is on the tach side. It has 3 prongs
and "just looks like it should be replaced", or suspected whenever the
things start getting wierd. They are cheap and I think Bus Depot has them
on the web.This easy possable fix beats commplex electrical diagnosis.
Nothin like  throwing parts at a problem,and hope they stick.