Auto Logout does not return to login window

I have a new install of Mac 10.7.4 on a 21" iMac. I have the iMac set to automatically log out after 14 minutes of idle time. On many occasions when the logout is triggerd, the user account will log out, but the screen does not display the login window, it still shows an image of the last state of when the computer was logged in. When I move the mouse, the screen acts as if it's erasing the pixels off the screen, showing bits and pieces of the login screen in the background. If I type a user ID and password, a small area clears around the login fields so I can see the fields and log in.

This was originally happening when a screen saver was enabled, I have since removed the screen saver. This did not make a difference.

I have the Preference Pane>Security>Advanced set to log out at 14 minutes and the Preference Pane>Energy Saver>Display Sleep set to go black at 15 minutes.

I have tried various different times/settings, but have not been able to fix this.

Since this configuration is for a Lab setting, I really need for the computer to log out cleanly and go back to the login screen.

Has anyone else seen this or found a remedy? I will try to grab a screen shot the next time this happens and post it to this thread.

Cheryl Tarbox

Macintosh Support Specialist

Binghamton University