Thanks, again, Thomas.
Thanks to your help on this, I now have a very good mental model of what's really going on under the hood with management of bridges in NM.
After fixing my network manager configuration, I now can isolate the problem I originally was looking for.
For some reason *dhclient* sometimes seems to die either prior to or during my desktop's "sleep/wake" cycle when it's not in use. Not always, just sometimes. It's clear it's dhclient, because the dhclient process is gone, but NetworkManager doesn't feel it needs to restart that process, when it brings up the connection after the wake.
Any thoughts on why this might happen or what to look for would be helpful.
If I figure things out, I''l let someone know (here if it's NM related -- it affects NM, in any case, but seems likely to be a dhclient problem).
David