Just wanted to let you know that I'm having very similar problems in the Auburn, WA area. The problem typically occurs after 7:30pm when I assume my entire neighborhood starts streaming 4K Netflix or something. Called Comcast support and got the same response you did: 3rd-party modem questions, connect directly to the modem, "we'll have someone come out" and no one showed (I was shocked this "ghosting" actually happened to me, as I'd always assumed people who said this happened to them were exaggerating). As you can see in my PingPlotter screenshot, the packet loss occurrs at Comcast's gateway and cascades down to the destination, making any streaming or realtime gaming a nightmare. Post here if you find a solution.
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