DNS dropping out intermittently, but Internet connection is good

I have been experiencing "no Internet connection" issues intermittently from many devices on my home network:  phones, computers, Xbox.  When I looked into it, I find that I can ping IP's on the internet, like 8.8.8.8, but trying a DNS lookup fails.   The DNS lookups fail against any DNS server, not just the ones configured on the router.

I know that this is not a real connection issue because some computers can say "no connection" while others are connected via VPN to a work location and have no issues at all.

My guess is that devices are trying to access a URL to determine if the "internet is up", so when the DNS lookup fails, they mark the internet as down.

I had several long sessions with Comcast.   First they sent out a replacement router.  Then, they sent out a tech.   He found several signal issues and changed out some of the splitters and rechecked connections.   That did not resolve the problem.

The dropouts can last a few minutes to over an hour.

Anyone have similar experiences?