mmmm .... what a relieve : finally found why my workstation was freezing at random... just, _don't_ try to use "hybrid SLI" with NVidia cards (it's supposed to gather the power of the discreet video card together with the MB embeded one.. ) !!
I don't know why I did enabled that in the BIOS, but it's crap and provoke xid 69 error (aka "hardware error")
now I can back to work .. (how many hours did I lost for that stupidity ... ? too many ... )