Nighthawk C7000 AC1900 how to test if modem/router vs Comcast issue?

All of our devices, both wired and wireless, are experiencing intermitten short access drops -- wired PC screen won't refresh, wifi Roku has connection delays (though it's just 3ft from router), etc. Then a minute later all will be fine. We have had a couple of moments where both wired PCs experienced delays at the same time when we were both just doing mundane browsing so we don't suspect the indiviual devices or wifi interferance. We have 200Mbps service, and typically get around 100 download speed wired, 50 wifi (using fast.com) which is usually fine for what we do (just two of us, never more than one movie streaming at a time, no gaming, etc.).

I'm trying to figure out how to determine whether this is an issue with the modem/router or with Comcast. All suggestions are much appreciated. I peeked at the logs for the router and didn't know how to make sense of what I was seeing.

FYI