Router interface unreachable!

I have a net of around 40 users connected thru’ 4 switches to a cisco 2600 router which is gateway. I have a seperate DHCP server running on windows 2003 server. Since recently the net works fine for 3 hours and computers are no more able to ping the router interface. But they ping each other and the DHCP server as well. I then have to reload the router and repair the DHCP server NIC and the net works fine again for 3 or 4 hours!
Any help..!?