It's been about 24 hours since the repair man was here (it's always the same guy, so that's kind of nice). I tentatively think it's all better now. I reserve the right to come back and tell you it's not, though, because that's the way my luck goes most of the time lol.
He replaced the cable and the radio transmitter. He did not replace the modem--that would have been the third new modem and he did not believe the modem was the problem. I agree with him, however, it would have been kind of nice to have all three be brand new so that we could eliminate all three if this begins happening again. I'll just cross my fingers that it doesn't.
I have had ZERO diagnostic codes and system state codes--besides the ones that tell me all is well. Before he came, every time I ran a connectivity test, I was losing at least 30% and usually 50%--sometimes 90%. Now I'm at zero.
Now that it works correctly, it's easier to see that it has been in a bad way for quite some time. It's so much faster now.
Anyway, I'll be back if I begin seeing errors again, but for now, it's working well.
Thank you to @Liz and the engineers for all of your help.
