Although this error is intermittent it is occurring on a network an effecting 24 users. When logged into a domain, already authenticated, the user locks their machine, comes back to unlock it and gets the message that they cannot be authenticated. They are forced to do a hard reboot, sometimes several times in a row before they can get logged back in. The other commonality here is the computers are all Dell Optiplex 380s. Any ideas or known fixes?