Help diagnosing video conference problems

PROBLEM

My setup has supported video conferencing well for several years, but starting a month or so ago I now have a recurring (but not perfectly consistent) problem where people I'm talking to say "we lost you for a few seconds" or "you dropped out--can you say that again?" It's very frustrating, and I don't know how to debug it. I don't assume it's an Xfinity problem, but I'm hoping someone out there can help me narrow it down. 

SETUP

My router is a Unifi UDM (not Pro.) My cable modem is an Arris SB6190. My Xfinity plan supports download speeds of about 500 Mbps
and uploads of 10 Mbps. I experience the same problem from several computers in my house, so the problem is not particular to one machine. One of the machines that regularly has the problem is connected to the router by ethernet, so the problem is not wifi interference.

Notably I don't see other people freeze. I can see and hear them the whole time. Also I stream movies often and don't have any trouble there. I suspect the problem affects outbound/upload data only. 

WHAT I TRIED

• I've patched/updated and rebooted all the machines involved. No change.
• I've run speed tests. Everything appears normal, uploading and downloading.
• I ping'd my router and observed 0% packet loss.
• I called Xfinity and asked them to troubleshoot. They said they ran some connection tests and said they saw no trouble on the outside.
• I looked at the Unifi dashboard to see if anything was hogging bandwidth during my calls--particularly upload bandwidth. Doesn't look like it.

I found a tool called "blip" that runs in a browser and pings constantly, reporting latency and packet loss. I ran it during a video conference during which the problem recurred. Blip does show packet loss at the times I freeze for other participants (though as I said they did not freeze for me.) I'll paste an example of the Blip output below.

How can I pinpoint the problem enough to see how to fix it?