Hmm, tried linux-lts. The Xorg.0.log looks quite different : https://gist.github.com/4524091. Also the sound is not working here . I should maybe try Xorg 1.12 instead of 1.13, shouldn't I ?
Thank you for any help.
YES !!! After installing package "catalyst-total" from yaourt, it works !!! Thank you everybody for your great help !