DNS Problems in my network

Ok guys I just took over an IT management position of a 25 computer network, but i have a major problem. I am constantly having to re-enter DNS entries on the Server 2003 DNS server we run. For some reason the DNS server seems to miss when the DHCP server gives out a different address than the machine had before. Secondly, certain webpages, like the domains that the company owns for example, will not load when you are inside our network. Outside the network it works just fine, and I can do a dns resolve to the correct IP address of the site, but internally I get nothing. I am at a loss right now, hope someone can help!