Bad connection or a fault to the unit.
Do you have it on a separate battery? It is best if they are, or connect direct to the starting battery, not connected via power that you find in the console or on a power bar.
Make sure you are using good quality copper stranded wire of the correct gauge with soldered or properly crimped connectors. I think it is 18 gauge, but your installation instructions will guide you. Also mentions 24 AWG. Manual states;
There are two wiring diagrams provided for you convenience. The first is a simple hook-up showing the GPSMAP 2006/2010 (using the 18-
pin wiring harness), the GPS 17 and the alarm circuit. The Alarm does not have to be wired for the unit to function. The second is a complete
wiring diagram showing the GPSMAP 2006/2010 (using the 18-pin wiring harness) interfacing with a variety of different equipment. The extra
5-pin wiring harness that came with the unit is intended for future interfacing features and does not require connection. Refer to the wiring
diagram that will best suite your needs. Make sure that all connections are good. For extra lengths of wire, 24
AWG (unless otherwise noted on
wiring diagram), shielded, twisted-pair wiring is recommended. We advise soldering all connections and sealing the connection with heat shrink
tubing. If you experience difficulty wiring the unit, please contact an installation professional in your area.
Make sure the connections are good on the rear of the unit including plugs to the transducer. You can coat with a little Vaseline which his electrically conductive.
States the unit is susceptible to RFI and should be 3 feet away from a radio antennae.
Finally, make sure the inline fuse and connections to it are good.
If all that seems good, the first thing to do is swap out the transducer for a new one. Still does it? Send the unit for repair.
Does it also do it at rest in the marina, or do you trailer the boat?
I have a Humminbird 998 HD Si. http://static.garmin.com/pumac/47_2006_2010_GPS17Installation.pdf