Are the lines in fixed positions on the screen - i.e. if you change the display resolution in System Preferences/Displays - are they still in the same place?
if they stay in same place and also no corresponding lines appear on an external display, then it's likely the LCD panel itself (and not the graphics chip)
there are a couple other posts with the tag [http://www.ifixit.com/Answers/Tag/lines+on+screen|lines on screen] where I previously mentioned possible failure of LCD driver ICs