At first I was only noticing it doing it maybe once or twice a day. Then it was more like every few hours. But this weekend it was literally happening like every 5-10 minutes!
So I went out today and bought a new cable modem. I've had my old once since I started cable modem service over four years ago. It's been running pretty much nonstop all that time. It's not beyond the realm of possibility that it's just plain worn out.
So far, so good. The conenction has seemed stable since I installed the new modem a couple hours ago. Hopefully it will stay that way. If so, it was a $60 well spent. :) I'm just glad cable modems don't cost the $270 they did when I first got the service!