DNS/SMTP

Here's the scenario. We're running Windows 2000 SP4. The main items of use on the server are IIS and SMTP, as the server hosts about 25 sites, some of which have feedback forms that use CDONTS or JMAIL to send the message containing the form results. When the server is rebooted, everything runs fine. After a few hours, though, the following happens:

1. I cannot open any websites via Internet Explorer.
2. I cannot get DNS results via NSLOOKUP if I use the "set vc" command, but I can otherwise.
3. No mail in the queue will send out.
4. CDONTS and JMAIL both error out saying that they cannot connect to the specified server.
5. I get messages in event viewer like: "Message delivery to the remote domain 'servint.com' failed for the following reason: An internal DNS error caused a failure to find the remote server."

Any ideas? Would something stupid like reinstalling SP4 help? As far as I know the server worked great before, so I don't think there's a hardware problem, but I'm willing to check anything and explore every possibility. Please feel free to ask any questions that may help clarify what's happening and help me get to the bottom of it.

Thanks!