Well, I tried to reset Unity and Gnome : the same.
I made fresh new install of 14.04 from a USB key : always the same.
When I boot 14.04 and choose "Try Ubuntu", the display is ok.
Attached is the log of "startx"
startx.txt
I tried removing gnome then reinstalling : nothing.
I don't know at what moment it happened but now, when I launch "startx", I get another log, see next attachment.
startx2.txt
I tried to find differences in the Xorg.0.log when it works and when it doesn't. When it works, I see lines about RADEON, when it doesn't, I don't see them. Apart from that, nothing. See next attachment for the faulty log.
Attachment 16208
I really don't know what to do now. The only thing I'm sure of is that my video card works.