Moblitz, thanks for those log entries. Those show intrusion attempts that happen to everyone but the number of intrusions is alarming but do you know if that's for one second, one week, or the number since the router was last restarted? If they're occurring in rapid succession then it could destabilise your router and cause it to lose Internet connectivity.
Google tells me that other Xfinity users have been complaining of exactly the same issue as yours. You may want to post on the Xfinity forums although I haven't seen a solution that's been confirmed to work.
Google also tells me that most Xfinity home users have a dynamic public IP address so if you can get a different public IP then that may fix your problem. Apparently, ipconfig/release then ipconfig/renew in a Command Prompt window should renew your public IP address, but if it doesn't, then turning the modem/router off for an hour or so should work.