Sounds like you have a low-level filesystem issue. Take a config backup before doing anything about it.
You might be able to use the console reboot menu to run a fsck (disk check) for the next reboot, if that doesn't work you'll need to reboot to single user mode from the loader menu and then run "fsck -y /" a few times until it finds nothing wrong. Don't stop running it when it claims the disk has been fixed, it needs to be re-run until it finds no new problems.
Worst case scenario, reinstall using the recover config.xml option to retain your existing settings.