After a few minutes of sitting idle my MacBook Pro's screen goes black. When you move the mouse the screen comes back to life, you see a bit of the screen saver, then it goes back to the desktop. If the computer is left to sit (until the screen is set to turn off) the sleep light will come on (steady, not pulsing). If the screen is set to never sleep, there is not issues. How can I fix this, it's really annoying.