Bumblebee problem: it sometimes freezes my laptop

I’ve a very strange problem with (I suppose) bumblebee on my Lenovo z70-80.
Sometimes after that kde is loaded, but before that steam is started (I’ve set steam to autostart after the boot), my laptop freezes. I can move the mouse but the cursor is really laggy and I can’t click anything. So I’ve to reboot using alt+stamp+reisub.
I’ve already tried to reinstall bumblebee a couple of times.
The very strange part is that this isn’t happening always, but only sometimes…

Any clue ? thank you very much