Need to start computer multiple time to get interface

Binary package hint: xorg

I bought a new Laptop : HP Pavilion dv6. I've installed Ubuntu 11.04 without any trouble (it was slow, but no error).

Now, to start my computer, I need to restart several time to get the User Interface. Sometime, I've to do it more than 6 time. In stead of starting, I get the an error (see image, sorry for the poor quality).

I can't explain why the computer start correctly after some reboot. I do nothing, just press the power button after the error, then, reboot. After doing this several time, Unity start.

The error told me about Radeon. So I imagine that it can come from :
- The New Xorg version
- The Free Radeon Graphic driver
- Unity
- Plymouth

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: xorg 1:7.6~3ubuntu9
ProcVersionSignature: Ubuntu 2.6.38-5.32-generic 2.6.38-rc6
Uname: Linux 2.6.38-5-generic x86_64
NonfreeKernelModules: wl
Architecture: amd64
CompizPlugins: [core,bailer,detection,composite,opengl,decor,mousepoll,vpswitch,regex,animation,snap,expo,move,compiztoolbox,place,grid,gnomecompat,wall,ezoom,workarounds,staticswitcher,resize,fade,scale,session,unityshell]
CompositorRunning: compiz
DRM.card0.HDMI.A.1:
 status: disconnected
 enabled: disabled
 dpms: On
 modes:
 edid-base64:
DRM.card0.LVDS.1:
 status: connected
 enabled: disabled
 dpms: On
 modes: 1366x768 1280x720 1152x768 1024x768 800x600 848x480 720x480 640x480
 edid-base64:
DRM.card0.VGA.1:
 status: disconnected
 enabled: disabled
 dpms: On
 modes:
 edid-base64:
DRM.card1.LVDS.2:
 status: connected
 enabled: enabled
 dpms: On
 modes: 1366x768
 edid-base64: AP///////wBMo1FBAAAAAAASAQOAIhN4Cof1lFdPjCcnUFQAAAABAQEBAQEBAQEBAQEBAQEBEhtWclAADDAwICUAWMIQAAAZAAAADwAAAAAAAAAAAB60AnQAAAAA/gBTQU1TVU5HCiAgICAgAAAA/gAxNTZBVDA5LUgwNAogAAM=
DRM.card1.VGA.2:
 status: disconnected
 enabled: disabled
 dpms: Off
 modes:
 edid-base64:
Date: Thu Mar 3 20:59:57 2011
DistUpgraded: Yes, recently upgraded Log time: 2011-02-19 22:42:17.234837
DistroCodename: natty
DistroVariant: ubuntu
DkmsStatus:
 bcmwl, 5.100.82.38+bdcom, 2.6.38-1-generic, x86_64: installed
 bcmwl, 5.100.82.38+bdcom, 2.6.38-5-generic, x86_64: installed
 bcmwl, 5.100.82.38+bdcom, 2.6.38-4-generic, x86_64: installed
GraphicsCard:
 Intel Corporation Core Processor Integrated Graphics Controller [8086:0046] (rev 02) (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company Device [103c:163c]
 ATI Technologies Inc Robson CE [AMD Radeon HD 6300 Series] [1002:68e4] (prog-if 00 [VGA controller])
   Subsystem: Hewlett-Packard Company Device [103c:163c]
InstallationMedia: Ubuntu 11.04 "Natty Narwhal" - Alpha amd64 (20110202)
MachineType: Hewlett-Packard HP Pavilion dv6 Notebook PC
ProcEnviron:
 LANGUAGE=fr_FR:fr_FR.UTF-8:fr:en_GB:en
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.38-5-generic root=UUID=b599fbd0-22d3-4a6b-8936-0e12224ef308 ro quiet splash vt.handoff=7
Renderer: Unknown
SourcePackage: xorg
UpgradeStatus: Upgraded to natty on 2011-02-19 (11 days ago)
dmi.bios.date: 10/21/2010
dmi.bios.vendor: Hewlett-Packard
dmi.bios.version: F.23
dmi.board.asset.tag: Base Board Asset Tag
dmi.board.name: 163C
dmi.board.vendor: Hewlett-Packard
dmi.board.version: 65.35
dmi.chassis.type: 10
dmi.chassis.vendor: Hewlett-Packard
dmi.chassis.version: N/A
dmi.modalias: dmi:bvnHewlett-Packard:bvrF.23:bd10/21/2010:svnHewlett-Packard:pnHPPaviliondv6NotebookPC:pvr0589120000242B10010020100:rvnHewlett-Packard:rn163C:rvr65.35:cvnHewlett-Packard:ct10:cvrN/A:
dmi.product.name: HP Pavilion dv6 Notebook PC
dmi.product.version: 0589120000242B10010020100
dmi.sys.vendor: Hewlett-Packard
version.compiz: compiz 1:0.9.4-0ubuntu3
version.libdrm2: libdrm2 2.4.23-1ubuntu3
version.libgl1-mesa-glx: libgl1-mesa-glx 7.10.1~git20110215.cc1636b6-0ubuntu2
version.xserver-xorg: xserver-xorg 1:7.6~3ubuntu9
version.xserver-xorg-video-ati: xserver-xorg-video-ati 1:6.14.0-0ubuntu2
version.xserver-xorg-video-intel: xserver-xorg-video-intel 2:2.14.0-4ubuntu1
version.xserver-xorg-video-nouveau: xserver-xorg-video-nouveau 1:0.0.16+git20110107+b795ca6e-0ubuntu5