I got that problem when I added the Down Imaging in January. I have the 798ci SI also. A solution is in the process at Humminbird. If you registered your unit, you will get the corrective software in your account when it is available.
As far as the cutting off, your battery is not strong enough to keep at least 10V going to the unit when the engine is started. It may be just corrosion on the battery terminals causing resistance. It could be a too small, undercharged, or weak-from-age battery. That is a common occurrence also and is not a Humminbird defect-it is caused by your boat battery maintenance.