Win 2K Client – DNS intermittent

All my Win 2k clients in an NT 4.0 domain have intermittent problems with DNS. My web server sits is a DMZ. Sometimes Win2K clients correctly resolve the DMZ address and can ping the web server. Sometimes they resolve to the external firewall address for our web server, (as though they are bypassing our internal DNS server and accessing our ISP’s DNS server) which they cannot access since they are inside the firewall. Since Win2K uses Dynamic DNS, I must be missing something in the DNS orClient setup? And ideas?