I just had to, with some embarrassment, leave the following explanation in closing out my service ticket with my (patient and wonderful) ISP, Speakeasy.
The context: My internet service has been bouncing up and down every hour and a half for the last day or so. Very frustrating.
I believe I've diagnosed it, and owe you a thanks for putting up with a customer-caused problem. When on the most recent phone call, I accidentally bumped into the power cable for my NAT box, rebooting the machine, which solved the mystery:
A curtain was brushing against the power cord. Over the last few months, the constant bumping had worked the plug loose to the point where it was just barely in. When the furnace would turn on, about every hour and a half, the forced air would cause the curtain to move -- which would glitch the power to the NAT box. After about 20 minutes, the house would be warm again, and the furnace would shut off... only to repeat an hour and a half later.
Thanks for your patience and help with this one. I'm going to go bonk the owner of said NAT box on the head for not looking at the uptime on his machine before crying wolf. I believe you can close the ticket."