1) Have you fully charged your battery and load tested it?
2) On the battery that your sonar is hooked to, is there anything else
hooked to that battery, like your main engine?
If so, when you start the engine, the battery draw may make your
unit cut off.
If possible, have your sonar hooked to it's own battery. It doesn't have to be a big battery.
3)You may try bending the connectors out a little on the power cable.
DON'T bend both of them at the same time or you will blow a fuse.
Even a slight jolt could jiggle the the power cable enough to cut off the unit.
4)One other thing that might be happening. You could have some debris in your power button.
Take a plastic (not wood, it could break), toothpick and insert it in the slot around your
power button and see if there is anything in there. I had some debris in mine that
caused the power button to stick.
Good Luck.