Thank you very much both of you for the help!
But now I have good news !
It seems it became stable now :o and I'm not sure updating the bios has anything to do with it since, it crashed once more right after (and almost instantly.)
It's been more than 20 minutes I'm running the kombustor OpenGL GPU burn-in bench at stock clock (power target is set to 100% although actual power is 110-117%, weird ?) and it won't crash eventhough I am trying to, by putting other windows above the bench (which would have made it crash almost instantly before..).
This is weird because I didn't change anything... It's still the same and latest nvidia driver.
I will try to raise power limit to see if it makes a difference.