Vertical Lines on Macbook Air Screen

I am having a recurring issue with my Macbook Air. It is a 13inch 2013 running OSX 10.9.5. This has happened more than once where I have gone to use my Mac and there are colourful vertical lines running down half of the screen. There has been no damage to the screen and nothing has been spilt on it, however I can make the issue to go away if I plug it into an external monitor. If i plug in a second monitor after a few minutes the lines disappear, but after a few weeks the issue comes back and I have to again plug in a second monitor to fix it. Any ideas what it might be?