windows 2000 internet looses internet

Hello I am having alot a problems with this 2000 domain I just took over .At first the clients didn’t have the lan dns server listed as dns server and it was taking 20 min to log them on to server .So at first I added the 2000 server as primary dnsand the internet provider dns as secondary .Computers logged right on but no internet.So I put primary and secondary internet dns in and added windows 2000 as extra dns in advanced settings.Worked like a charm but after couple minutes I lose internet or if i reset computer I loose internet.The computers where getting ips and dns from DHCP on router before so i tried going with static ips still same thing.Any clues to what I am doing wrong?