"Do not rely on this product"

I have already done a hard reset

since you are questioning the motion of the boat causing the problem,it leads me to believe you have the power cord wired to a switch?? or is it directly wired to the battery??
either way,are you still using the original fuse and holder???

I only ask because I have seen where this kind of scenario has happened and it ended up being either a bad ground,bad switch,bad fuse holder or any other place an existing and reused connection is at fault.
I believe you when you say Lowrance found the first unit being bad,but I have to ask is there something to do with the wiring/boat/connection that is causing the problem seeing as how it keeps showing up.
I have seen failed diodes in the motors charging system,or a failed battery charger cause this kind of problem as it induces A/C current into a D/C device frying it.

try this,with the boat sitting in the driveway and the unit powered up,carefully,gingerly,wiggle every inch,and I mean every inch of the wires starting at the battery up to the fuse holder,(wiggle the fuse also) and switch if used,this includes the connector on the back of the unit.
if you can duplicate the problem doing this,try and isolate the spot where it is happening at and repair it.

best of luck and let us know what you find out in case someone else has the same issue with their unit doing the same thing.
side note: I have owned lowrance products for many years and have never had a complaint with their service.
sheldon

You might create an overlay that shows voltage on the screen. I had similar flaky issues on my bow HDS unit so I displayed the voltage and it was 10v when the console units read 12v. I was going to rewire power to the bow with fatter wire but I accidentally discovered I had a bad connection and re-seating the connectors fixed it.