Raymarine Seatalk NG network not talking. Sometimes!

[barrett bondon]

Buddy of mine bought a new used boat with a VERY complex Raymarine Seatalk network. It works - and then fails.

All stations go out at the same time.
All stations continue to get power.
All data to all displays is lost.
Sometimes it will all come back up in moments. Sometimes minutes or hours or not at all.
And there are a LOT of displays. The boat has four complete nav stations so there are multiple I70's and I60's and many other displays - with all the wiring very well hidden and painful to expose.

Any thoughts on a likely cause of this failure greatly appreciated.