workstation webpages stop working after a day or so windows 2008 server

I have setup a windows server 2008 as a fileserver with 7 users. The problem is every day or so the network cannot load webpages but the system remains on the internet. When I restart the server or restart DNS then it seems fine. I have changed the router but no help.

Router = 192.168.1.1 (NO DHCP)
Fileserver = 192.268.1.2 (DHCP and DNS SERVER)

All workstations connect via DHCP

I have also multiple onboard NiCS on the server but I am only using one of them

any help is really appreciated

We had a problem many moons ago where the DNS Servers at the provider changed after a period of time and caused a similar issue.