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Having this same issue. Been doing a serious ammount of troubleshooting and testing myself as Comcast wants to charge me to get a technician to help, even though this only started happening since a tech came out and installed the two cable boxes.
Anyone have any tips?
Start here;
https://forums.xfinity.com/t5/Your-Home-Network/Connection-Troubleshooting-Tips/m-p/1253575#M94474
Post the requested info from here;