Update -- I got a call from Comcast this morning from someone who said he had done something to fix the problem, which was apparently not specific to my home. I'm really tired (from getting up early for the HVAC guys) so I didn't press for details, I just said thanks very much and cancelled tomorrow's appointment.

I have to say that I'm surprised that they (a) had someone look into it, rather than just waiting for a tech to come to my house, and (b) called me to follow up. If only they were this good when I call about problems with my DVR, and they tell me that someone will call me within 48 hours but no one ever calls.

So, fingers crossed that whatever the guy did really fixed the problem for good.