Unable to access Web Sites after a few days externally

We have a 2003 Web Server Edition running a mixture of PHP and .NET sites. They randomly go down and external access to these sites stops working. The W3WP.exe processes are still running and we can access the site from the web server ok using the internal IP address. We are running DebugDiag but no userdumps are being created as nothing actually crashes. We are getting desperate now as we cannot find what is causing the failures. A reboot fixes the problem.

Check the server event logs. There are some aftermarket IIS diagnostic programs as far as I know, and perhaps one would help you isolate the problem. Carefully review your PHP installation and code too, I once had a problem with an older PHP that would cause IIS on a Vista computer to stop after a few hours, even with no web activity. But the server wouldn't show php pages locally when it crashed either.

Maybe you have some other firewall problem, or some time-based restrictions setup somewhere. For example, in my cheap soho router I can set port forwardings to only work at certain times and not work at other times.