"Morgans" wrote

It could be that it is not putting out enough voltage/amps while it is
under load.

To further expound on that point, you need to know what the power draw is
demanded by the unit. Take a thin piece of cardboard, and put a wire on one
side to contact the positive battery contact, and run it to an amp/ohm/volt
meter, then back to the contact of the GPS unit, to be able to read the amps
(or milliamps) actually drawn from the batteries.

Of course, the output of the ships power adapter must be equal to or greater
than the draw of the GPS. I really think you will find your problem, there.
--
Jim in NC