Server logs out overnight!

:shock: Why would our 10.1.5 server automatically log out overnight? It started doing this a while ago then we found our root password had been changed. So we plugged all incoming traffic from our router, reinstalled the OS, turned on its firewall, left all sharing services OFF, disabled root user, and it STILL logs off overnight-- I'm greeted with the log-in screen in the morning. Please help!