Well not sure I'm happy or not to have found this thread but it does appear to explain my issue exactly... have to hard reboot the CGN3 every day or so after the internet connection dies a slow death. When it is up the speeds are amazing (I'm getting 326/21 on Ultimate) but clearly there are issues. I've been pulling my hair out the past week trying to diagnose given I also installed a new router the day before the new modem (Netgear R7000).
I swapped out the modem the first day as I had an issue where it would not go back online after a reboot... but otherwise my symptoms are exactly the same as the rest described here. Any suggestions on how to deal with Tech Support (aside from another useless modem swap) would be appreciated but it doesn't sound like there is a fix available yet.