Thanks ghostbunny, i'll try it as soon as possible.
I got more information: it's a Venturer LCD TV 19". So it's not really a computer screen, maybe it's why I got some problems...
I try it at my home on a PCLinuxOS Openbox and it works (but not perfectly as it flash all 20 seconds).
With Sabayon OS on the same computer, it's all right.
When I'll retry with her computer, I will try to set the screen with XFdrake, but I'm really not sure if I'll be able to resolved the problem like this...