Fedora 18 starts in "alternative" mode desktop

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http://forums.fedoraforum.org – Since some day ago, my pc has started in an alternative mode desktop when restarting after a system update How can I have again gnome3 working? Why Fedora intermittently becomes unusable (to say the least)? Fedora18 Núcleo Linux 3.8.6-203.fc18.x86_64 GNOME 3.6.3 HARDWARE: RAM 1.9GB CPU: Intel® Core™2 Duo CPU T7300 @ 2.00GHz × 2 (HowTos)