Howdy…
I have an older 788 CI. Last weekend we found a piece of structure that was holding some nice Crappies and we decided to drop a waypoint on it. As soon as I touched the “Mark” button, the unit unit shut down and restarted. It did it several times in a row, after shutting the unit down, checking supply voltage, etc. Not expected behavior! Later on I discovered I had the unit set to CST, not CDST so the GPS indicated time and the time set in the unit differed by an hour. Yup, as soon as I set it for daylight savings time, it worked normally again and I had no trouble setting a waypoint.
Yet another reason why DST is one of the stupidest ideas ever conceived by humans.
SR