Daily, intermittent internet disconnections and high packet-loss spikes (not on wifi, multiple routers and no router also tried)

I have had 1Gig internet through Xfinity for over a year at this location. I rented a Xfi gateway for the first few months of service and then purchased my own modem (Arris Surfboard S33, DOCSIS 3.1) and router/access point (Ubiquiti UniFi Dream Machine). The first few months of using my own hardware were totally fine- no issues with latency and no reported outages in the router's log. However, in the past two months I have begun experiencing issues both with total network-wide internet connection dropping (even on hardware that is connected via Ethernet) and very high spikes of packet loss and latency (x2-5 a day, seemingly random times, and from anywhere from one minute to a half hour long.

I have reset my router and modem settings multiple times, turned off all port forwarding and traffic management rules, tried a different brand of router, and even eschewed the router and directly wired a test computer to the modem and these spikes and outages still occur. It is most evident when online gaming and using VoIP/Wifi-calling but any activity that requires high data transfer (video, streaming, etc) also suffers. I can provide router logs if exactly timestamps are desired but there are no recorded outages and each time the Comcast-side diagnostic shows a healthy network.

No cables around the house have been disturbed and all connections appear to be tight and in good condition so I have exhausted troubleshooting on my personal network. I would rather eat my hat than sit on through a couple of hours of tier 1 tech support before this issue is escalated. My best guess is that either some cabling or connection in the area is damaged or a piece of Xfinity hardware (a server?) is misbehaving.