New DS/DNS Server 2003, slow performance and user rights`

We have just installed server 2003, created our domain, installed DNS, and configured the users.

Problem is, after an hour or two performance on the network grinds to a halt. Some users log in very slowly, others have problems connecting to a drive that they have Admin rights to, and for others the internet stop working. (I can ping server and router, but if I try, say, yahoo.com, it doesn’t resolve).

This has to be a DNS configuration issue but I can’t seem to figure it out.

Any help would be great.