I would start by checking all the connections on motor cord to plug, then wiring connections from batteries to plug on dash, then connections on all wires at batteries. Sounds like your losing some voltage some where. Also pull head and check those spade connectors under ther. They can loosen up from heat over time. Another thing if you don’t find anything obvious would be bypass circuit breaker and go straight to battery. That way you can eliminate breaker as the problem or find it is dropping voltage.
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Charlie Weaver USN/ENC 1965-1979