Problem access internt using DNS in firewall.

Hi all, I have setup my firewall as my DNS server. Lately I have problem browsing internet from my LAN, it seem my DNS server down and cant ping to any host like google or yahoo. But after trying couple of times, I can access that site again. My router and modem is working fine. I still can do nslookup from the server to yahoo or google. It only happen for 1-2 times and sometime we can easily acces the internet. It's like my server cant hold all access to internet, but I only have 50 PC's accessing internet. Anything I should check ? maybe in Firewall or anywhere ? regards, -- Arie Reynaldi Zanahar reymanx at gmail.com http://www.reynaldi.com