Thanks styler for this very good tuto.
In the meantime, I have solved my problem. I replaced the old usb cable (which was electrically ok) with a shielded usb 2.0 cable, and everything works fine. So BEWARE OF THE CABLE QUALITY WITH ARDUINO 
Thanks styler for this very good tuto.
In the meantime, I have solved my problem. I replaced the old usb cable (which was electrically ok) with a shielded usb 2.0 cable, and everything works fine. So BEWARE OF THE CABLE QUALITY WITH ARDUINO