Display troubles on a Toshiba Satellite

[Yalio]

Hi everybody,
I have a "little" problem with an old Toshiba Satellite on that I've recently installed Lubuntu (12.04): there is no way to use the GUI correctly. At startup, when loading, the screen flashes with a lateral shift of the image. Next, a black screen occurs instead of the GUI.

If I go into text mode (Ctrl + Alt + F1), the display works. By connecting a VGA output on the screen, I can get better results : the GUI is visible... but again, there are continual flashes (in this case, also in text mode)...
I changed the resolutions and refresh frequencies several times, but it doesn't fix the problem. I also looked at the "additional drivers" module. It seeks about ten seconds then told me that there is no proprietary driver on the machine.

I looked on the Toshiba website, I didn't find any drivers for my own model. Before Lubuntu, Windows XP was installed on that computer and there were no graphical problems.
I nosed too on the (french) ubuntu wiki, and I found a few tracks, but now I'm a bit blocked...

I tried the following command which is supposed to print some informations about my graphical device...

$ lspci -vnn | egrep "VGA|3D|Display"
00:02.0 VGA compatible controller [0300]: Intel Corporation 82830 CGC [Chipset Graphics Controller] [8086:3577] (rev 04) (prog-if 00 [VGA controller])
00:02.1 Display controller [0380]: Intel Corporation 82830 CGC [Chipset Graphics Controller] [8086:3577]

I looked too about the Xorg config. The /etc/X11/xorg.conf file didn't exist, so I created it. Now, I confess it, I'm a little lost about the way to fill it, despite of the documentation page I found (in french).

Finally, I looked at the xrandr command. When I called, it returns me a pretty (or it is ?) "Can't open display", while the built-in laptop screen actually works.

Does anyone know how to fix the problem?

Thanks a lot !



P.S : I'm French a I have translated this post from an original one, written in french which doesn't received any answers.
So I apologize for the grammar mistakes which can appears in the post here.