Well, it looks like the tech that replaced the line to the house was right about that being the problem. It’s been up almost all the time since he did that about 3 weeks ago. I have no idea why no other Comcast techs could figure that out, though.
Now, there’ve been other issues that weren’t related to the same problem. I couldn’t get to the Internet on several different occasions, but that was always a problem further upstream. The cable modem was always fully connected and I could ping the closest hop in those cases.
And then there’ve been the times when I couldn’t get to the Internet, but turning the cable modem off and back on fixed it. Again, those times it still had all the lights on, so it wasn’t the same connectivity issue as the last few months, but it’s a sign that something weird is still going on. This is probably a network-wide issue, though, rather than just being me.
On the whole, much better than before. That’s not going to stop me from looking into FIOS, though.