Same behavior in my case. I am using OpenDNS.
The only factor I have noticed is that it happens usually during heavy usage (high loads) but I still haven’t figure out the exact reason.
I do not use ZS for DNS for now, requests are forwarded to a local bind9 server which in turn requests from OpenDNS (220.127.116.11 and 18.104.22.168). But still I have the same error randomly.