Having intermittent issues with Zoom, but possibly other programs as well - downstream issue?

We are having what appears to be a suite of issues.  Basically, it appears something downstream from my house is causing a spaz out of my connection 3-4 times an hour.  This is becoming very apparent as my wife is on Zoom a lot, and it embarassed by how much it errors out while she's on. I'm decently technical, and have done all i know to do.  But before calling Xfinity tech support, wanted whatever advice from people here to facilitate my discussion.  Here's the details:

First the techinical stuff:

A) I ran Speedcheck.net tests on all manner of devices, wired and wi-fi.  Uploads of 18 MBps wired and wireless, dowloads of 500 MBps+ wired and 300+ MBps wireless.  Definitely much more than needed for our purposes

B) I'm using the Xfinity wifi router directly - had been using an Apple Time Capsule + wifi router.  Decided to cut the fat and run everything without the extra layer.

Here's the issues at play:

1) Zoom: several times an hour, my wife gets a pop up that says "internet connection is unstable".  She teaches, and its VERY disruptive.  We've tried different places in the house, wired, different devices (my MacBook, her MacBook, my work Windows laptop, various iOS devices) and they all have basically the same issue of timing out a few times an hour.  Phones did best, but still break up about every 5 minutes.

2) Similar happens on WebEx

3) During this process, I've realized many apps/programs seem to have a similar behavior.  YouTube vids will appear to hitch at about this often (restarting the app fixes it), Magic the Gathering Arena gets a "Network Error" about once every 10 minutes (again, restarting the app fixes it), and my work VPN (used to connect to a dataserver linked to Excel PowerQuery) will give a connection error and require me to re-start Excel if it sits idle for more than 10-15 minutes.

4) We never have issues with streaming - Netflix, iTunes, HBO, etc all work flawlessly - I suspect b/c they're able to buffer, while the stuff above has a "real time" element and can't buffer.

These all FEEL similar now that I think about it.  If anyone has any ideas for me to check and/or language to ask a help desk tech about, please let me know.