Intermittent SERVFAIL from different locations - how to diagnose and fix?

Solution 1:

^{expanded from my earlier comment:}

Thank you for posting your actual domain. That allowed me to check your current settings.

On thing I noticed was that the TTL, the time-to-live of your DNS record was set to 300 seconds.

300 seconds, 5 minutes, is quite a low value, which most people only choose when preparing for a change of IP-address or for example as part of a fail-over strategy.

Unless you have a good reason for having your TTL at 300 seconds research shows that increasing TTL values generally reduces latency and will make your DNS more robust. See for example sidnlabs.nl/en/news-and-blogs/how-to-choose-dns-ttl-values

For general zone owners, we recommend longer TTLs: at least one hour, and ideally four, eight, or 24 hours. Assuming planned maintenance can be scheduled in advance, long TTLs have little cost.