Intermittent NXDOMAIN for a name we are forwarding

Sat Feb 20 01:08:56 UTC 2016

We have a DNS setup where we forward a name in one domain to 5 external nameservers. We see NXDOMAIN error intermittently (once in couple of weeks). How do I debug this issue?

I took a cache dump on our DNS and 2 out of 5 nameserver IPs appear in "Unassociated entries" when the problem happens.

Any advice to troubleshoot this issue is greatly appreciated.

Thanks
Blr