When I leave iTunes open, my computer crashes..

When I want to listen to music, I open iTunes on my MacBook Pro (Mac OS X Lion 10.7.5). I leave this program running while I am doing whatever. When I come back to my computer either because I want a different song or the album has ended, the screen will have gone into power saving mode (black). To get the computer to wake back up I wiggle my finger on the touch pad. By doing so the computer then goes to a light gray screen and begins to restart. After I have the computer reloaded it tells me to submit a crash report (which I have done numerous times. I have no idea what the crash report is supposed to do.). How do I get this to stop happening?

I have run both tests recommended from the "Using Apple Hardware Test", but both test came back with "No trouble found." So I'm a little lost. Please help. Thanks!