I don't think your solar panels
are contributing - they are just passing DC current
. I don't know enough about the D400 to say where the EMI may be occurring.
Have you tried putting ferrites on the power leads of the Furuno
and D400? Also, the power cable for the Furuno
has a shield that should be tied to ground at one end.
I don't think it is a problem with the Furuno - it does sound more like a GPS
problem. When it is happening, what does the GPS page on the Furuno tell you about the satellite
number and signal quality and the HDOP?