Im having the same issue with same model and xfinity. When tech was st my house we were hitting faster 400Mbps speeds, but by end of the day, speeds again dropped to 25Mbps.
Xfinity says its my modem as well. it is 4 months old. Im wondering if there was a firmware update released and either xfinity ddnt test it or has not applied it.
I cant find where the latest firmware should be.
Since you are experiencing with intermittent internet connection, let us know whether the issue is with wired connection or wireless connection.
If its wired intermittent internet connection, I suggest you to check the cable signal levels.
To check the same, find the steps in the below given link:
http://arris.force.com/consumers/articles/General_FAQs/SVG2482AC-Cable-Signal-Levels
If you are experiencing wireless intermittent connection, change the Wi-Fi channels by following the given link.
http://arris.force.com/consumers/articles/General_FAQs/SVG2482AC-Changing-the-Wireless-Channel