I have had a 2011 macbook air 13" since mid 2011. Have had no problems with it, haven't dropped it nor damaged it in any way and still looks like new. But recently the screen has been flashing from normal screen color to negative screen color. The screen will stay negative unless you move the screen into the right position then it will switch back. This is very irritating as the warranty on it has just ran out. Any ideas on what could be wrong would be much appreciated.