it seems to work now !

i tried what you suggested without reinstalling windows (it was a fresh install for Z77+2700k) and it did not work

so here's what i did next :

- flashed to bios 801
- turbo @40 and vcore @
- reinstalled windows
- reinstalled asus drivers
- flashed to bios 906
- put turbo at 40 and vcore at 1,35, removed speedstep and voila !

i did a little test @x45 just to confirm, it's ok now.

i guess it was a bad bios / corrupted OS combo
thx a lot raj !