Did you watch this?
There is a known issue with the CM1200 and Xfinity. I also have this modem and with LAG enabled I pull 1200down / 45 up however after a period of about 6 hours the connection starts drops speeds to 25/25 until a modem reset occurs. Netgear can't fix it because xfinity has to push the firmware update. can we please get a reply on this? its been a issue for over a year and there are hundreds of others maybe thousands by this point.