Reoccurring brief DNS outages - where's the problem?

Lately my home internet service has been experiencing brief periods of failure and I'm trying to understand what the root problem is and if there's anything I can do to fix it.

I have two piholes running on my home network providing DNS service and blocking. Lately I've been encountering brief internet outages, approximately once a day for the last week. Work laptop disconnects from VPN saying no internet and other devices show no internet either. Cable modem doesn't show it's lost connection though, and logging into my router it shows it has internet connectivity. Trying to ping a website shows the dns resolution isn't working. I can ping by IP address but not name. The outage usually resolves itself in approx 5 minutes, sometimes a little longer.

My piholes are setup to use Google, Cloudflare and Open DNS for dns service so I'm puzzled how my dns resolution suddenly starts failing. In theory if one's down, it'll use the others right? So how am I experiencing a dns outage? Is this a problem with something outside my network or something internal, or is there anything else I should do to diagnose and fix it?