I had a very similar problem, in rough water my graphs would turn off. Turned out to be the master power wire pulling out of the crimp at the battery. I shook the wires like you with no result but when I tapped on the connection, at the battery, with the graph on, the screen flickered. Bingo.There is also the ground block mounted on the transom where you might have a loose connection or corrosion. Something is not making solid contact, and Im betting it is a loose crimp like mine. Because of the rough water and flickering.