The sender for an electronic meter such as this generates low level signals used by the gauge, much like the electronic pickup in your distributor, and as such, is subject to noise disturbances, and other unwanted currents, such as 'ground loops' in the wiring/ chassis/ engine.
make sure you are using shielded wire, and try grounding the shield at ONE END ONLY of the run.
Run the wiring neatly along the body, clipped as close to the body as possible, because the ground of the unibody helps form a signal ground.
It may be picking up noise from the ignition or alternator, or simply have a ground loop from one end to the other.