S3 was still down this morning, so it was a good excuse to take the new boat out for a spin. ifdown eth1/ifup eth1 got the network back up and all was well. As usual, data were saved locally, so no data were lost.
Despite the fun, this is getting old, so I've now installed (updated and put into crontab) the net_check script on every system in the marsh. Previously, it had only been running on S8. As another test, I did this all on eddy which is now running VPN (thanks, Gordon).