MacBookPro shuts down on its own

I have a OS X 10.6.8 on an old macbook pro. I'm not sure if this problem started after I did an OnyX cleanup. If I'm away from the computer for a long time, instead of just going into sleep, it almost shuts down, the screen saver is there, but all previously open applications are closed, and it's starting up when I wake it up. I don't know what that is or why it happens, as it's not in any of the energy saving setups (copmuter is set to never go on sleep).