When you got this computer, did you by chance restore anything from a backup that was from another computer?
When I got my MBP, I restored a time machine backup that was from a 2008 Mac Pro. The restore worked and everything functioned. Only problem was that my iTunes purchased videos would stutter and the screen would remain black for the first 45 seconds of the video. After this, the video would play fine.
Yesterday, I started out with a fresh install of SL and then restored my settings from my Time Machine backup. Videos play fine now.
I would make sure you have a complete backup of your computer and then try a fresh install of the OS. Something sounds corrupted. I have the same MBP as you do, 2009, 15" with 4GB ram.