So, I have a second-hand Humminbird 788ci with external GPS puck, installed at the end of season last year, so minimal use.
Every few minutes I will get a few beeps, and the depth gauge changes from actual depth (like 4-6 feet) to something ridiculous like 89 feet for a couple seconds, then goes back to correct depth.
My father has a similar unit that does the same thing.
What can be the cause of the interference? I haven't used a stopwatch to see if it is evenly spaced. My father has autopilot, I don't - so can't be that. Happens to me in Eau Claire, to him up in Solon Springs and on St. Louis River. I have a Mercruiser I/O, he has a Honda 4-stroke.