Seeing as the IPv6 only stays up for 15 minutes at a time thread is now locked for some reason, I am posting here.
As mentioned by other Gigabit customers using the Technicolor TC4400 modem (still the only option at the 940Mbps service level) there is an odd bug affecting the IPV6 WAN address which causes the modem to drop the address after 15 minutes or so.
After calling support the suggestion was to swap modems. I will be doing that this week and I will update the post.
The previous thread was locked because it was posted by a different author in June and was resolved. While your issue is similar, the best way to get support and responses from your peers is to post a new thread. Please check back and let us know if swapping the modem resolved the issue.