While checking ports, I discovered ip address 104.18.20.226 popping in & out. WhoIS.com resolved this as Cloudflare.

Does anyone know why this would be connecting to my Desktop every now-and-then?
I could block it in my firewall. But, what's on my desktop that would do this? I can't find anything.

Do you use 1.1.1.1 or 1.0.0.1 DNS Servers ? That might do it.

It is a possibility that someone that uses Cloudflare has something misconfigured and has the IP address defined in a DNS entry.

Maybe you could contact abuse@cloudflare.com and ask them if they could investigate it.

I had at one time used the CloudFlare DNS. But have changed it back to Xfinity or Google on all my devices.