I've found a workaround. Adding i915.i915_enable_rc6=0 to the kernel options in grub makes all work fine.
This is my hardware : CPU INFO processor : 0 vendor_id : GenuineIntel cpu family : 6 model : 42 model name : Intel(R) Core(TM) i5-2400 CPU @ 3.10GHz Base Board Information Manufacturer: Gigabyte Technology Co., Ltd. Product Name: H67M-D2 BIOS Information Vendor: Award Software International, Inc. Version: F7 Release Date: 11/26/2010 VGA INFO: VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09) (prog-if 00 [VGA controller]) -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xserver-xorg-video-intel in Ubuntu. https://bugs.launchpad.net/bugs/978286 Title: [12.04] [sandybridge]freeze after login in greeter To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-intel/+bug/978286/+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