I hope they didn't drag things out for you...
Sonic support was very responsive and put a lot of time into working with the line (I had Fusion X2 service, over copper). The line would work awesomely for a while, but then, on the line graphs, I could see where the sync rates on line 1 would go crazy. Sonic technicians visited my house several times, Sonic and AT&T ran diagnostics many times, Sonic and AT&T did vendor meets, and so forth.
The worst part is the issues with the erratic sync rates were intermittent, mostly occurring April through October. From November through March, the line was mostly stable.
Sonic tests showed AC current was present on my line, and theorized that it might be due to the Panasonic cordless phone I had hooked up. We agreed, however, that the cordless phone uses DC current--that's what the wall wart provides to it! This was verified when I left the phone unplugged and the current was still detected on the line.
Techs tried typical steps such as limiting the bit rate, and yet at some point, intermittent noise would occur, and then it would resync even lower. We finally agreed that no matter how low the cap, when the noise occurred, the resync would be lower than the cap. so we left it uncapped. I learned to check the sync stats periodically and record them, so that Sonic would have the information. I got into the habit of forcing the modem to resync multiple times a week. Most of the time, it would immediately sync at the usual rate, but sometimes I simply could not get it to sync and be stable.
One trick that I learned was to unplug everything: the MPOE test jack, the DSL filter, the phone line to the modem (at both ends), and the phone line to the phone (at both ends). On more than one occasion, this seemed to get it to sync when it was having issues. I started wondering if the various connections might be corroding, and unplugging and plugging might be cleaning the contacts. <shrug>
One day, out of frustration, I grabbed another filter out of the box and plugged it in, and for several weeks, the line stayed stable. When it went unstable again, I tried changing to another filter, and subsequently, the line was stable for another few weeks.
BTW, AT&T swapped pairs several times, and even did a port swap at the CO, and the issues persisted.
My theory is that somewhere along the path for line 1, the copper got close to some sort of equipment that threw out a strong EMF field, used extensively during the summer, and occasionally in the winter... perhaps a pool or hot tub pump, or something similar? I was about 4,500 feet from the CO (although occasionally Sonic's test equipment suggested that I was 7,500 feet--I suspect it sometimes got confused by whatever was causing the problems).
Sonic never gave up. Unfortunately, their hands were tied by using AT&T's infrastructure, so some dispatches depended on the latter's availability, and that wasn't during a pandemic when many more households were demanding instant service. And AT&T is trying to abandon its copper network, so some technicians were more helpful than others.
I've since moved from that house. At my current home, I'm on IPBB X2, and I'm about 300 feet from the VRAD. I can trace the line its entire length, and I can verify that it's not routed near anything that should be causing noise on the line.