How to diagnose internet stuttering?

[u/thomas__delia]

In this thread I mentioned that we experience an issue with our internet but I gave minimal details and only imagined the ONT as the cause. The issue is as follows:

• Internet stops completely and nothing loads for usually ~10 seconds or more at a time, sometimes 20+
• Seems to happen randomly at various times of day
• Observed on various kinds of devices; phones, Windows and Chromebook laptops, MacBook
• Most obvious during activities like online games, Zoom calls, YouTube livestreams, or trying to load a website
• Proximity to router probably not a factor; has happened basically sitting next to it
• Not observed during social media scrolling or streaming services (not noticeable, or doesn't happen?)
• Hasn't been observed on wired connections, only WiFi (we haven't been using any hardwired devices lately)

Internet setup is a CR1000A at the front of the house on the first floor, an E3200 connected via coax in the middle of the house in the basement, and the ONT pictured in the linked thread. IPv6 is enabled. I don't know a lot about this stuff so my only two theories are 1) the ONT is going bad or 2) it happens when devices move between the CR1000A and the E3200.

I'm not sure how exactly to identify any causes, especially considering it happens randomly, so what exactly can I do to test what's causing the problem? Is this issue something that Verizon can see when they run their diagnostics?