When running an Xfinity Internet speed test, particularly when the uplink is being tested, all channells on all 4 TVs will pixelate. In previous visits Comcast has replaced all coax connectors in the home and upgraded Comcast hardware to latest X1 equipment. Power cycle on all equipment does not fix it.
Most likely your modem's upstream signal is at or near the maximum limits. I've seen this happen at my parent's house where Comcast put in one of those active return amps that supposedly compensate for splitter losses. The high upstream seems to overload the amp.
zero gain amps remove need for 'high upstream' power.
My main coax feed from the outside comes into my basement and first hits an amp "Extreme, Infinity Promise, model IPA1008DL-RSVF, 52-1002 MHZ forward gain 0.0db, 5-42 MHZ reverse gain 0.0db". It has 4 of the 8 outputs in use, going to 4 separate TVs. I have a Comcast visit scheduled for today.