Further trouble shooting confirms that the erratic tach is heat related. Driving at night, or on a cooler (for Tucson), overcast day the tach is fine. On a hot day, or after leaving the car parked outdoors, the tach starts acting up.

I'll add "replace tach" to my wish list.

has anybody tried to replace the gauge with an aftermarket unit but switch out the faces. probably wouldn't be that straight forward but i figure somebody has tried it.