Please select one of the following items to troubleshoot: It is very important to have the gauge and the sender ground, grounded to the same exact ground screw.
A small ground loop may accure if the gauge ground is under the dash and the sender ground is in the engine compartment.
This may cause the reference ground to be at different levels and the gauge will jump usually up when an added load is on the electrical system.
Do not have multiple grounding points, have one single ground point where both devices are grounded to.
It makes no difference if it is close to the gauge or at the sender ground as long as it is one single point.