Tom
maybe it's the hosting provider's DNS failing us, that could be the culprit.
I experienced that. I had lots of issues with a hosting provider where I had a website, I couldn't even Ping or ICMP them, but to them everything appeared normal.
This all ended when I changed hosts.
For another website I changed to a hosting provider that has more DNS servers (many cheap ones just provide 2), that seems to have fixed all my issues.
Dont try Cloudflare btw, that will only partially solve this.