can't shutdown

[mvtlam]

Sometimes, when I try to shutdown my computer, instead of shutting down, the login screen appears.
Then, if i try to shutdown again (without command line, juste GUI), nothing happens. And if I try "sudo shutdown 0" on command line, the exit screen appears (with "ubuntu" and some white dots bellow), but my computer doesn't shutdown, the screen just stays like that.

Sory for my bad english.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: gnome-power-manager 3.4.0-0ubuntu1
ProcVersionSignature: Ubuntu 3.2.0-24.38-generic-pae 3.2.16
Uname: Linux 3.2.0-24-generic-pae i686
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu7
Architecture: i386
Date: Sat May 12 14:58:26 2012
InstallationMedia: Ubuntu 10.04 LTS "Lucid Lynx" - Release i386 (20100429)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: gnome-power-manager
UpgradeStatus: Upgraded to precise on 2012-04-01 (40 days ago)