Mike, didn't you have a problem with an Smiths electronic
oil pressure gauges as well? It does sound like it's a grounding problem.
Please note....I'm not bagging Smiths, as all my gauges are Smiths gauges (mechanical), and an electronic Smiths tacho. I wouldn't use anything else.... although I do also have a mechanical SW fuel pressure gauge.
Good luck with the fault finding.
Cheers,
Glen