This will take a long time to check out, fordem. After all, even wireless, this mapping was only lost from time-to-time, and several weeks could pass without the mapping's being dropped at all (although it could also drop two times in one week). To illustrate, my mapping to the DNS-323 has not dropped since I posted my request asking for an explanation! When it does, I shall hard connect to the router, per your suggestion, and try it out for a month or two.
Incidentally - although it may well be related to this problem - I have recently changed my AV-protection because my response times had deteriorated to utterly unacceptable levels. Since that change, my response times have reduced dramatically - specifically, to something between a tenth and a fifth of what they were. Here, by "response time" I mean the time between entering a command (eg, clicking on an icon, or on a web page) and when the command is finished executing.