Most likely it's the lcd panel. A while back a friend had a 2007 MacBook Pro with a similar problem. I took the display apart and put some layers of tape where the ribbon cable connects to the panel, so that there would be pressure on the cable when the screen was put back together. It didn't totally get rid of the problem, but it happened much less frequently.
I'm not too familiar with the 2008 model and if the screen can be dismantled.