Welcome to the Humminbird Side Imaging Forums minerdoug.
First, check your unit to see what software version your 596c HD DI unit running. Update it if needed as I believe that version 6.690 is the latest. Be sure to reset the menus back to the factory default settings by using the Restore Defaults menu.
Do you know of anyone else that has a Humminbird DI model unit that you could cross-test units with? This will help determine whether the problem is in the unit or transducer/boat.