Not trying to step on anyones toes here. But the best way to test is to wire the FF to its own battery. Most power problems on a DC circuit will turn out to be a faulty connection of some sort. Slightly corroded fuses , loose wire terminals etc etc.

My old 997 used to go black when I started with cranking battery. Finally got tired of living with and wired to its own battery. Problem vanished. I realize it is suggested to use cranking battery as your power source. But I suspect it is an electrical engineer making this suggestion. When I was working as an electronic tech we used to have a saying. Engineers design it but they leave it to us tech s to actually make it work.

To simplify you have three actual possible failures.

1. Bad Unit
2. Bad Battery
3. Bad connection related to terminals , fuses etc etc.

To fix problem you must determine the root cause. The easiest and fastest is to eliminate the battery as the prime suspect. I have a spare 12 volt tractor battery that I use as a trouble shooter for friend and relatives boat/fishfinder woes. Starters for outboards pull a lot of amps. If the pull is hard enough it might trigger a low voltage protection circuit on the circuit board of the Humminbird.

If this happens every time you start the motor and the Humminbird runs fine the rest of the time. I can almost assure you that wiring it to its own battery will solve problem.