Same behavior in my case. I am using OpenDNS.
LAN—>local_bind9_DNS_Cache(OpenDNS)—>Zeroshell—>ADSL_Router—>INTERNET
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 (208.67.222.222 and 208.67.220.220). But still I have the same error randomly.