I just got a new TV set it up fine but now some channels the picture gets pixelated and the sound comes in and out. If it were the cables wouldn’t it have happened with the old TV on the same channels?
check the connections on your cables for tightness (coax). if all seems tight, you will need to call 800-Comcast to get a tech out to check at least outside your home for signal/noise levels. I don't know if they are able to come into your home in your area during Covid-19.