DHCP OR DNS Problem ?

Hi, I recently installed win2k server and I am having a very annoying problem and i believe it is something simple , I just cant seem to find it. I have installed DHCP, DNS and AD. In the beginning everything was fine , but it seems my clients will lost connectivity to the internet all together after a while or after a couple of reboots. Why is this ? I can still ping and resolve the DNS names of both internal machines, but why would they lose internet connectivity. I have checked the DHCP lease to infinite and nothing helps. I even tried pinging with ip and I still get host unreacheable. Any help would be greatly appreciated, Sincerely, Carlos!