In support of the ground issue theory, I had a gauge that wasn’t lighting up with my navy lights. I took the wiring apart, measured current, made sure it SHOULD all work, and cleaned everything.
Still didn’t work.
Ended up spraying everything with CRC and covering connections in dielectric grease and magically things started working.
I think the punishment of marine environments on electrical connections can make them extremely finicky.
I’d suggest turning on your VHF so you’re hearing loud static and then go from the battery up to the helm shaking cables especially near their connections.
Given that you’re having intermittent power supply issues, that should help you identify your problem spot:
Don’t ask me how I know
Posted from my iPhone/Mobile device