Same problem here, please let me know if resolved. There is another post here where a guy went through (3) modem changes, changed all cables, no luck. After weeks, Xfinity finally determined there was a problem in one of the outside boxes up the street. Intermittent problems are the absolute WORST to troubleshoot !!
I may have slowed it down considerably, it could be purely a coincidence though. I logged in to my xfinity account and enabled the XiFi security option and have not had a problem since. That was the Monday night (3-2) I beleive. The true test will be this weekend when everyone is home.