Spontaneous Restart/Logout

My MBP is exhibiting this type of behaviour and I am wondering whether anyone else has experienced this and has a solution/fix.

It happens like this: The screen saver is activated. After a time (I'm guessing about 45min - 1hr) the computer logouts out of my account and the logout screen is displayed.

Is there any reason why this would happen? I've checked my System Prefs and they don't indicate anything of that sort, unless my not checking the right place.

Would anyone be able to help?

Thanks,

Ed