Yesterday mid-afternoon, I came home to the internet down again, which is more normal than abnormal. I called Comcast and explained that I was setting up another internet provider and wanted to know the soonest date I could be out of my contract. Comcast snapped into action. They got a Comcast employee here in 90 minutes. Not a contractor like the last 10 times, but a Comcast employee. According to Comcast records, every technician had reported that they fixed the problem. None had. The last said it couldn't be fixed.
The Comcast employee who came yesterday did a quick but extensive rewiring job. The internet connection has been perfect since he left. That's only about 18 hours, but the internet has never worked without interruption for more than an hour, so I'm pretty sure it's fixed.
The technician said he found bad connections and a crimped wire that would have drastically affected the signal. When he left, he said that he's been doing this work for 18 years, and "everything I know from experience says that was the problem." He apologized and said he didn't know how the contractors could have overlooked it. He speculated that they were lazy.