I am seeing the same issue on the GNC 355. It seems to appear more often when there is no other traffic anywhere in range (Canada does not have ADS-B ground stations like the US so I get weather, notams, etc. via SirusXM). I wonder if there is a shielding issue on the Ethernet connection that makes it sensitive to electrical noise?
I have the GTX 45R and the firmware update is possible as the windows software to update it via USB (USB connector is under the panel just behind the USB power) is available on the Garmin
site. I need to install a Windows VM on my MacBook Pro and see if the USB will connect.