bind forward to rbldns server intermittently fails

[Ramprasad]

Hi
I am using bind 9.2.4 on Centos 4.2 
We run a fairly  high traffic Antispam service with 6-7 million mails
hitting a day ( 14 load balanced servers each serving upto 35k
mails/hour ) 

For checking antispam , we use a "local" rbldns server running rbldns on
port 530 and every scan server is configured to forward all rbl-zones to
the rbldns server

so in my named.conf I have 
zone "xbl.spamhaus.org" IN {
 type forward;
 forward first;
 forwarders {
 192.168.2.2 port 530;
 };
};

The hits on the rbldns server would be around 1000qps 

Everything seems to work perfectly fine .. but sometimes all of sudden
the named server seems to have failed When I "dig" for a listed ip I get
a negative response. I restart bind and then for the same ip I get
positive response

Any ideas how do I debug this 

Thanks
Ram