yes i'm sure it's Windows 7 (x64) ^^
i think i've maybe found the problem.
I've update my wifi driver from ralink website since i stop freezing.
But not totally sure cause it's random..
My wificard is 'ovislink', in first time i've use their driver who seams work correctly but not in reallity.
Today i've take the last driver form ralink and all problem seems to be resolve.
Let's see in few days.