I had the same problem with an early 2011 15' MBP within a month of purchase date. Apple replaced the display panel and the problem returned. They then replaced the logic board and the problem continued. Then they "made adjustments and corrected a cable" but the problem would not go away. Finally the store manager replaced the unit with a late 2011 15" MBP. I never did find out what caused the problem but I was happy with the final resolution.
I suggest you take it to the Apple store and have them check it out. BTW, I had 3-4 different pics of the problem that I showed the Genius guy and the manager during the process. I think that helped my case. You might want to do the same. Good luck on getting it resolved.