I have a problem and have gone through three c7800. My current understanding falls into two potential issues that remain unresolved from NETGEAR or COMCAST.
1. The device does not prevision correctly during the authorization of the device on the network.
2. The device is in some way not managed correctly by COMCAST.
In either of these two cases, you will see a connectivity issue when traffic is placed on the device. I simulate this by using a few laptops, tablets and have kids use stuff that stream. What happens next has me at a loss. The device appears to lose the internet not the WIFI just the internet. I say appears because on future examination it lost IPV4 connectivity and not IPV6. This can be tested using (https://test-ipv6-vm4.comcast.net) it's clear as day. What the user sees is many sites don't work. However, as more popular ones use IPV6 (google, amazon, facebook, etc.) you never even know you are in this state.
The major concern for anyone working from home is SKYPE for Business and OUTLOOK (office 365) use IPV4. Which has a nasty result of connection issues with work meetings!
I have this escalated to tier 2 levels however as the line between service and hardware is so grey, both NETGEAR and COMCAST don't recognize the issue.
This remains unsolved if you can help there is a $50 dollar Dunkin donuts gift card in it for you. As I had to buy another $200 modem.