Since the A75 was the last addition, and seems to correspond with when the problem started, it seems the most likely cause or trigger for the problem. The other possibility is that something else failed at the same time, but that seems much less likely.
I would guess that the A75 is transmitting data that is also being transmitted by some other device. And this has caused the issue. Changing all your device instance numbers probably worked around the issue.
Note that Maretron is the only vendor who it super sensitive to device instance numbers. They require unique instances to select between any like devices, e.g. GPSs, depth
sounders, etc.. Other vendors select sources based on a combination of the device's function, model, manufacturer name, and the device's unique serial
number. So they can select between two GPSs for the same model and same vendor based on the serial
number. All devices instances can be zero and everything will work. But this isn't true with Maretron.
Sorry for the side bar, but N2K Instances are greatly misunderstood.