I have the same setup, OP, I also started having problems similar to yours, it was the Comcast entry cable. The techs came out and after an hour determined the signal level was too low and the entrance cable needed to be replaced. In discussion with the techs, we think this was due to a close lighting strike. As noted above the cable box (DVR and tuner) as well as the Xifinity modem should stay on all the time. I will occasionally reboot these just to make sure they are up to date, as I recall these check for updates when they are powering up.
IMHO, complain to Comcast, if you get lucky they will come out and check signal levels.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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