Random Intermittent DNS Failures—Need Help!

I'm experiencing some random, intermittent DNS issues in our setup that prevent access to a couple of internal sites with external DNS records, specifically server1.company.com and server2.company.com. We have three AD DNS servers: two located in the Data Center and one over an IPSEC tunnel at our office for redundancy. Replication is functioning properly according to 'repadmin /showrepl'. This problem started suddenly, with no changes made by our internal IT team. We also have BGP routes to a larger corporate infrastructure, but since our DHCP only supplies local DNS, I don't think that plays a role.

Each DC is configured with a primary DNS set to another AD DNS and secondary set to query itself (127.0.0.1), which I believe is standard practice. Our forwarders are set to 1.1.1.1 and 8.8.8.8, and root hints are active. The issue is that randomly, the DNS servers fail to return results for the external DNS records for the mentioned servers—during these occurrences, I did a dig from various servers:

dc3.company.local returned a SERVFAIL, while dc1.company.local had no problem returning the correct info. This seems to happen intermittently across all of our AD DNS servers, where typically at least one works while another fails.

To diagnose further, I've run 'dcdiag /e /v /test:dns' and the summary showed some failing tests on our DCs. This fail could pose problems since we have an Isilon setup that requires DNS delegation, which is mentioned in the test results. Any guidance or insights would be greatly appreciated!