Welcome to the Humminbird Side Imaging Forums Bobbylove.
About the only thing that you can do is check to see that you have a good electrical connection to the battery. Even though you may be getting 12+ volts to the unit, it may not be getting the amperage it needs to work correctly. Check all electrical connections for any that are loose or corroded. Be sure to check the fuse, fuse holder and battery connections too. Getting the battery load tested is always a good idea as well. If the power cable has been lengthened be sure that the wire size is at least 18 gauge if not larger. Better yet would be to try connecting your unit to someone else’s boat that has a Humminbird Down Imaging model unit and see if it does the same thing on their boat or not.
Otherwise you will need to contact the Humminbird Customer Resource folks again and set it all up to get the unit in to be fixed.