It is hard to say. There are three possibilities: the LCD, the display cable, and the logic board.
The display cable could be the culprit if it is getting pinched around the hinges. This, however, usually gets worse with time and is affected by the angle of the hinges.
It can possibly be the logic board video chipset going screwy but its rare. A good test is to hook it up to an external monitor and see if the problem continues there. This isn't a 100% test, but does help narrow down the possibility.
It is most likely going to be the LCD from my experience with any sort of lines or banding across the screen. You are looking at $350 for the screen and an additional $95 if you want us to install it. It's not a terribly difficult installation if you have any experience with the internals of a computer.