See if this helps:
@steve757 Try the troubleshooting steps in @RobertWy reply and see if that helps. I've been having issues with Gigabit tier on DOCSIS 3.1 myself due to outside ingress from presumably another Gigabit subscriber that keeps either knocking me offline off and on for a few minutes or hours at a time or puts so many errors on the line it locks my modem up at 35 Mbps until I reboot it. Of course I figured this out on my own and the furthest I've gotten with actual Xfinity techs is they will put my modem on a 24 hour monitor to gather information and that was a week ago with no positive results since. Any time they test my line, this other presumable Gigabit subscriber isn't using it so it tests on their equipment fine. I suspect it has something to do with the OFDM channel that DOCSIS 3.1 utilizes because a lot of my issues happen when that line ingress knocks my SNR on OFDM below 30 dB. Meanwhile, I can connect via WiFi to a neighbor's Xfinity hotspot and it will be completely fine at the same time, presumably because it's still using DOCSIS 3.0 on a lower speed tier without OFDM.
Hi there, steve757. Was the issue resolved? I wanted to make sure before I close out this post. If not, I would love to help.