My car was stored for a long time and I've had a bunch of minor electrical issues when I put it back on the road.
I've a tach w/ shift light that behaves weird now:

When I drive, it reads about 500-600 rpm too high, then suddenly snaps back to a regular reading, then back to high, etc.
Then a couple days ago, I had the contact on, but the engine off and the needle could decide between 0 and 600 rpm...
Obviously there was no signal for the rpm input, so I have to rule out noise / bad connection / ground, etc.
Has anyone ever seen that? btw, shaking the wires, hitting the thing, etc doesn't change anything, so it doesn't seem like something loose.

 

I'm not certain of the exact cause, but with any electrical problem I always start with the grounds first.