I had the same issue a while back where my CGN2 in bridge mode would slow to a crawl then a reboot would fix it for a while. At that time it was an issue with the rogers network in my area. The routes(internet paths) kept changing and seemed to leave open connections on the modem/router. That could be your issue. Record traceroutes when the service is good and the service is bad and compare that path taken on the rogers network.
I have also read of a capacitance condition of the cable feed itself causing similar issues. The line will gradually build up a small capacitance charge and the reboot (or a disconnect of the cable from the modem) will discharge it. You would need a skilled tech on site to diagnose thise condition though.