Macbook pro 5,2 Falls asleep unexpectedly

My 17" MacBook Pro 5,2 has a problem with staying awake... At random (increasingly so) the screen will flicker and it will put itself to sleep...

After I wake it up again, I'll see growl notifications about magicprefs (wich is standard after awaking my macbook). Does anybody have an answer as to how to keep my macbook pro from falling asleep every 2 minutes? Thanxzzzzzzzz...... Huh?