It seems to have gone away again. Here are some impressions I have on the problem.
If I start up this computer without starting out its associated LAN, it seems to show up.
I had my system set up to use the boot ROM for the onboard LAN port. When I disabled the boot ROM, it also seemed to show up.
I have not attempted to reproduce the problem yet because I'm afraid of making things worse.
If I can't boot up the system because GRUB isn't working, how do I reinstall it?