It turned out, that the graphics card was shutting down because it got to hot. A proper flow of air was impeded because the holes of the tower were not completely free.
** Changed in: xorg (Ubuntu) Status: Incomplete => Invalid -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xorg in Ubuntu. https://bugs.launchpad.net/bugs/898761 Title: Display falls into sleep after a bit of use in Unity but not in Unity 2D To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xorg/+bug/898761/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp