Bind does not resolve host names an returns error.

[schnell-im-netz Gmbh - DS]

Hello


We use a Debian Box with BIND 9.2.4 
Bind is hosting nearly 800 Domain.
But sometimes, our Dialup User cant resolv Hostnames.
Bind responses with and Servfail.
When i restart the Server the problem is solved, but why 
does it stop resolving some domain?

Has anyone had this problem before?
What can be the problem?

Options are
options {
        directory "/var/named";
        listen-on port 53 { xxxxxxxxxxxxxx };
        check-names master warn;
        pid-file "/var/run/bind/run/named.pid";
        datasize default;
        stacksize default;
        coresize default;
        files unlimited;
        recursion yes;
        multiple-cnames no;
        notify yes;
        //forward first;
        transfers-in 10000;
        transfer-format many-answers;
        max-transfer-time-in 5;
};



Dominik Sennfelder