'You are not authorized' message appears until complete reboot

We have a few NT boxes running Domino and on occasion, these boxes require a complete reboot because no one is able to access them remotely for administration and can't even perform a file-database-open. It comes back with the "You are not authorized" message. After rebooting the server, it works. We checked the server docs and can't see anything wrong, don't see anything wrong with any deny access lists, ACL's look fine. Any suggestions why this happens?
It sounds like your server is running out of memory, and therefore cannot process any more requests. I would look to upgrade the server code to 5.0.9a, and buy some more RAM. Also, look to see what is leaking. If it is an agent causing your problem, you can fix it.