I had the same problem after install in the Gnome 64 version. fixed itself after installing proprietary ATI driver.
What I did:
Accept license and use defaults.Code:wget http://www2.ati.com/drivers/linux/ati-driver-installer-11-5-x86.x86_64.run sh ati-driver-installer-11-5-x86.x86_64.run
Restart system.
If you don't want the proprietary driver, maybe this will work:
Code:dpkg-reconfigure xserver-xorg



