Has Cox provided you with information about what IPv6 settings that you should use with their service? For example, here is a document that Comcast has provided to their customers. The exact same settings are correct for Time Warner customers as well. Things have been rock solid for months using these settings on my AirPort Extreme here with Internet service from Time Warner.
http://mydeviceinfo.xfinity.com/download.php?file_id=35
I'm not saying that these are the correct settings for Cox......because it appears from information in other threads that Cox may be implementing IPv6 in a different manner than other providers are using.
I think we at least need a starting point from which to work......and that would be some instructions and direction from Cox about what settings that customers should be using on their routers for IPv6 service.
We've seen some posts about using a setting of Link Local Only for IPv6 with the Cox service......but this simply disables the IPv6 service from the Internet...and customers are receiving no benefit that way.