I have a macbook and it is 5 years old. The backlight of the monitor is temperamental. I know it is the screen, because I can still see that the screen works when I look at it from an angle. So therefore it is not the logic boards:-)
Also the macbook works fine with an external monitor.
Now I was wondering could this be because of a connection of the monitor to the screen or is it time to get a new monitor?