I have been running Ubuntu 12.04 LTS on my Dell Optiplex 755 desktop and ACer laptop for a year or so with few issues. Both run very well now. I am installing Ubuntu 12.04.4 onto my brother's desktop, wiping out Windows XP, and reconfiguring the partitions all from a bootable USB drive. The install went well. I set up his username and password, installed Gnome shell to replace Unity. The system will boot, let me log in and then it may be (1.) unresponsive to the mouse -or- (2.) Be unresponsive to mouse clicks when I move the mouse to try to do anything (3.) After a few minutes it seems to have some type of GPU issue and reboots to the Password screen. After that it only shows the Unity Background. No task bars at top or bottom of screen. Do I have a GPU conflict with a 9400GT? The on board GPU does not work at all. Any guidance would be appreciated.