As before, I called Comcast to advise of this in advance of the tech's arrival. The person that I got today suggested that the tech test the line back to the box, which has been done twice now, as well as the modem (ditto). Then, he wondered if the problem was that I was on an old Comcast internet package.
I have sincere doubts that would have anything to do with it.
But the tech will be out here tomorrow morning.