Single point grounding means one common point (physically) say for neg power out of your bridge console. If you have different ground points, there could be a potential difference between the multiple points thus the chassic grounds are not operating at the same level for each device, and can circulate current between them.
If you are not sure if the Garmin is OK, send it back to the factory along with the entire harness, power plug, etc, and ask them to verify the unit is in compliance with noise standards it was certified / built to.
Modern electronics are under higher industry noise standards than older equipment was. This should not occur any more.
I sent my Raymarine equipment back to the factory three times, each time they said nothing was wrong with any component. The head unit would shut down and restart by itself, showing a scanner failure - loss of radar, like I was going out fishing in fog all day with this garbage. System was less than 1 season old.
Even though they tested the scanner fine, I knew the scanner was not integrating properly, so out of frustration, I bought a new scanner and it fixed the problem immediately, paid $900 plus tax to fix the problem even though it was all under warranty, I had no choice. Need I have to say where my money is not going next time.
Also check your FF cable to be sure the outer braided conductor at the plug is making good contact, and the plug is fully engaged.
By the way, do you have any inverters, battery chargers, or things with motors running when the interference occurs ?
And are there times with no interference? If so, what's the difference about the circumstances?
