A Tohatsu dealer probably would not blame motor grounding for your issue. But again, if we don't know what/how they tested, they are just guessing and giving you a voodoo answer.
Don't know about this motor, there should be a wire ground connection betwenn swivel bracket and lower leg, probably one ground terminal broke, there's too much corrotion, salt build ups on it and not doing its homework right for anodes to work properly.
