Good info. I hear bad things between the MB8600. I think they are stripped down for price savings, and Cox got a big inventory of them on sale, so that's why they are pushing them. AFAIK there was a firmware issue around the ethernet bonding of port 1-2. Also, some people had issue access the modem's diagnostic if using a router with certain IP range.
In general, it seems like you have a large house. How many ethernet jacks do you have in it? What are you using for a switch/panel? Is the CAT7 shielded?If not, I wonder if that could be throwing anything off. Sometimes non shield ports can ,mess with the line duplex. However if firmware is all that fixed it, I wonder if it had something to do with IPv6. Have you tried disabling that?
