Likely cause of electronics interference

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.

Incorrect depth readings at less than 15 feet is a common problem on Humminbirds. Go to your settings and set the surface clutter setting to 7, and the medium/83 kHz sensitivity offset should be set from -4 to -6 depending on need. 

Swede, check your messenger. I have a ton of cards laying around.

I need either a 32GB card, or the patch cable to do the update.  Thinking the card is going to be cheaper and easier to obtain.  I have the latest update downloaded and waiting on my laptop.....just need a way to transfer.

All 3 wires (external GPS, power, and sonar) are run together from the back of the boat up to the unit on the dash. 

Ulbian's post also suggested the transducer cable could be causing it.  Gonna have to splice in an extension to run that wire up the opposite side.

Do you have the latest software version installed?

It can come from sonar wires and power wires being ran close to each other as well. Why the sonar wire isn't shielded is beyond me.

I have a love/hate relationship with HB.

Just an update - have not made any changes, but now I get about 20 seconds of interference, 5 seconds accurate reading, then 20 seconds interference, etc.

Perhaps I have stumbled upon the reason why it is a second-hand unit?

On my boat, I use Humminbird,& Lowrance side by side on the dash,& a Lowrance on the bow with no interference issues. All hooked up to my accessory battery.

Mine is running off my starter battery.  No connection to the trolling motor batteries (24V system) at all.

I believe my father's is set up the same way.

I do have a second sounder on the boat, a simple Garmin Echo 200, but the issue did not stop when I shut the Garmin off.  Transducers are on opposite side of the motor, and I went into the settings to select frequencies that do not match.

Are the units powered off of the same battery as your trolling motor? If they are change that. Run them off of your cranking battery or a designated accessory battery. Just don’t have them on the same circuit as your trolling motor. 

There could be a shielding issue with the wires on the ‘birds. I’m not as familiar with those as I am Lowrance but if they are not shielded properly other wires running along side of the transducer cable can cause funky stuff like that to happen.