I have had the same issue for many years now. Comcast techs come out to the house replace the cable under the house and the issue continues where the TV signal blinks on and off, sometimes staying off for periods as long as 30 secs. The only thing that helps is resetting the box, plugging and replugging AC to box. However there are times when the issue will return only a few moments later after resetting the box. It doesn’t matter which port the box is connected to and I remember a thread mentioning weather changes can cause this, something about frequency. Not really sure what I can do about the weather.
It’s so annoying to pay so much for service and never have this issue fixed. This issue is not isolated to the living room cable box, we also have one in the bedroom and that one seems to flicker more often. Sigh.
Does your TV not have the issue with an antenna as its source?