You must enter in safe mode and install graphics 4800 driver from multibeast.
Or detele the content of itunes (this worked for me)
try booting with flag -x -v
Well, it seems that all of my problems come from my GPU : GeForce GTX 970.
It seems that it is not Mavericks compatible.
And I'm stuck to Mavericks due to software compatibilities.
So it seems that the only way for me is to exchange the GTX 970 for a GTX 780 or something in the 7xx series ?
Windows download tool makes the best USB installer with least problems, but Windows machine is required.Ok thank you !
For the moment I installed Yosemite using Clover and it's running ok.
I'm now struggling to make a bootable USB installer for Windows 7.