The SAME damned problem just reoccurred. I immediately got on the phone with the tech support guys at my ISP and did some troubleshooting on the phone. It turns out that my problem is probably a weak connection on the back of my PC. A little judicious wiggling of the ethernet cable on the back of the PC made the problem go away...for the moment.
Another great thing about small-town life: I'm on a first-name basis with the tech support guys at my ISP. They're friendly, helpful, and above all, knowledgeable. It is so VERY cool to be able to make a phone call, get connected to tech support by a real live person, and have your problem seriously considered. No "make an appointment," no damned voice response systems with a menu that would confuse Einstein, no hassles, no problems.
Life is good.