Horizontal Lines On Screen

[coolikedat99]

I'm trying to fix my granparents Gateway NV53 laptop. The only thing that is wrong with it is that the screen has horizontal lines on it. The lines change (they are usually white, grey, or black) positions and color on the screen while in use. When I connect it to a display it I can see the normal picture on the external display. But the laptop screen still has lines running through it. I have tried starting it up in safe mode. It didn't work. I have taken out the screen and am waiting to find out whether it is a bad inverter, or a bad screen or what. Any information would be helpful.
P.S.
By the way, the computer turns on and everything works fine except for the display.