Hello tmj327

                      If this is happening on the devices connected to modem Wi-Fi and on all devices connected to Wi-Fi then we would suggest you to try changing Wi-Fi channel on the modem interface, If other Wi-Fi sources such as neighboring wireless access points are using the same wireless channel, this may cause intermittent connectivity issues, even if the competing signal is relatively weak. Changing the wireless channel within the  gateway can improve connectivity. please follow the steps mentioned on the link below, After changing Wi-Fi channel as recommended on the link restart the modem then i would request you to monitor the connection for next 24hrs.

                    http://arris.force.com/consumers/articles/General_FAQs/G34-G36-Changing-the-Wi-Fi-Channel

Let us know if this helps

Yes, I had already done this with some longer periods between interruptions, but the interruptions are still there.

Xfinity always says the internet is fine on the first check. 

do you have a copy of the cable connections page and event logs? 

I could probably get those, but the frustration of having to deal with this is ridiculous right out of the box when it is recommended by Xfinity as a preferred router/modem. I’m going to return it and get something that can handle the ups and downs of Xfinity’s internet better.