dns stops working after x time

Hello,

a weird situattion happens on our 2008dc
after a while the dns server stops worling correctly
when we do a nslookup, the dsn doenst answer.

the dns server service is still running though, even a reboot doenst get it
to work properly
when i reboot the server, the dns service is running fine again for a day or
2 to stop working unexplainably.

Any tips?
Tx

C:\Users\Administrator>nslookup www.google.be
Server: pcvo-server.pcvomeetjesland.local
Address: 172.16.0.1

DNS request timed out.
timeout was 2 seconds.
DNS request timed out.
timeout was 2 seconds.
*** Time-out in de aanvraag aan pcvo-server.pcvomeetjesland.local