Hi Katarina,
This shutting down like this could be because of a number of things, it's not and SL issue though i'm sure. as there is so many reasons it can do this i think you should take a look at the link below and as you will see it could be hardware or even a virus causing this to happen, anyhoo have a read i hope it helps.
Good Luck.
http://www.computerhope.com/issues/ch000689.htm