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.
On 8/14 problem was isolated to errors coming in from the pole. I've opened 4 tickets since to get the Comcast pole crew out, each one gets closed after 48 hours with no visit. I'm on my 5th ticket / message in a bottle. All 4 TV's unwatchable. Family is NOT happy with me. Sorry, but no other place to vent.
Comcast line crew ( not contractor ) came out and found multiple problems ( signal leaks, misbalanced connections, and loose connections ) on the junctions up on the pole outside my house and a couple more down the street. I'm near the end of a long run, the Comcast hut which converts fiber to coax is a pretty good distance down the road. These problems were causing the errors. The line crew made all the fixes and now so far so good, no more pixelation and internet appears faster ( probably because now most packets are error free and don't have to be retransmitted ). So, after 1.5 years of an intermittant + gradually getting worse problem, I am cautiously optimistic that it is fixed. REMINDER ..... if you have intermittant pixelation on Comcast TV, and you also have Comcast Internet on the same cable coming into your house, run an internet speed test to see if it trips the TV pixelation problem - it's a usefull troubleshooting tool for Joe Consumer.
Well, it was fixed for about 2 montha, but now the problem is back ...... use the Internet and TV pixelates. So I started the process all over again, called in a ticket, tech came out, problem coming in from the pole, trouble "given" to Xfinity cable crew / pole folks. I've learned that the communication between the main trouble support center and the pole folks can best be likened to a message in a bottle. Of the 10 previous times I was told that it has been "given to the pole folks", I would say 3 times it actually made it to the pole folks, the other 7 "givens" were lost at sea. However, I have the cell of the last pole person who fixed it and I just texted him myself. Comcast despirately needs to develop a true two-way communication / ticketing system between the customer support mothership and the pole folks / local engineering. It's 2019.
From all the previous issues, I had the cell for the Comcast outside cable crew, I texted the guy who fixed it originally 2 months. He graciously came back out again to check it out. He found a new signal leak a few poles down and repaired it. He then tweaked some pole aplifiers again and said the signal was perfect leaving the last pole. I went inside, ran an internet speed test and the tv still pixelated. Long story short, I bypassed the Comcast amplifier in my basement, creating just a single home run to my main tv and NO Pixelation. Looks like the comcast amp is the problem. Outside cable crew engineer said he would contact an inside supervisor to have the amp removed - and some other solution put in to serve my 4 TV's ..... once in a while you get shown the light.