Intermittent connectivity issues

I had Xfinity in CA for 5 years with zero problems. Moved to UT, upgraded to Gigabit (I had 250 in CA), and it has been nothing but problems.

I do a lot of video conferencing for work from my home office, and I frequently get dropped connections and "your connection is unstable" messages in Zoom where my screen freezes and people can't hear me. Sometimes I'll do a speed test right after this happens and I'm getting ~900 Mbps on my iMac that is connected via ethernet. Other times, like today, I'll get 25-30 Mbps. So it seems like the issue is intermittent and not always related to download/upload speed—or at least it drops out and reconnects fast enough that by the time I do a speed test it's okay again.

I have a desktop computer connected via ethernet. Our house is wired, so the XB6 gateway is connected to a switch, and my desktop is plugged into the ethernet port in the wall. I also have a laptop that is connected via wifi. I experience this problem on both the desktop and laptop / ethernet and wifi.

I've reset the modem, no effect. Xfinity technician came out, replaced the cables, checked connections, said everything looked fine. I have another visit scheduled for Sunday, but not much confidence that they'll be able to identify or fix the problem because it's intermittent. For example, just did a speed test and got 930 down and 40 up. Connection "seems" to be working fine, but inevitably, if I'm doing a video conference for 30 min or more, I'll get at least one (and often several) freezes/drop-outs. 

Any ideas for how I can further troubleshoot? Thanks.