That said, it's time to look at fallback plans. (And, ultimately, we may want to move to a service that will support higher speeds in any case, but I'd prefer to do so in my own time.) The two main alternative options seem to be Verizon FIOS and Cox cable internet; does anybody have any rants or raves for those services?
Thanks for any useful tips.
ADDENDUM Tuesday Evening: The new DSL box was waiting at home. I installed it; after an initial failure it worked when I pulled and re-planted the phone and ethernet lines. So far, so good, but given that the problem was an intermittent one it'll be a while before I know whether replacing the box is going to make a difference.