Just to close the loop one this (for now), I want to share the status. I cleaned the battery and leads, reinstalled the battery and drove to the local Volvo shop. While they had the car (2 weeks, at my request) the problem never recurred. After the first week they wanted to send the car home, but I asked them to keep trying and to use the car for errands, etc. After 2 weeks of "good health" I accepted that I'd just have to take my chances driving the 90 or so miles a day. (BTW, I'm up to about 174,000.) The only work they performed was to pull the "possible" parts, clean the connections and return the parts. I've now driven the car another three weeks without the problem recurring. Keep your fingers crossed that things stay that way. Thanks to all for your guidance.

Also, I have a new (and hopefully unrelated) problem. If you're interested, see the new post titled: To Cruise or Not To Cruise

Be safe...and Keep Pedalling