I'd look into the wiring. I know when i helped install an aftermarket tach, when the coil wire lead was loose, the tach jumped around, but only when the car was on. With it doing it when the motor isn't running, i'm not sure. but the first thing i'd do would be make sure the connections are tight, and there are no wires that are half way in. That's caused me a couple headaches in the past.