[thews]

The first thing I'd do is beef up the ground. Electronics need a common ground to work. If the ground is intermittent or not solid, your gauges won't know where circuit common is. Get a braided wire and find ground on the gauges. Run that point to the body and make sure the connections at each point have a clean surface. Once that's as solid as you can make it, finding a problem will be much easier to diagnose.