In descending order of probability:

Bad LVDS or eDP cable

Bad panel

Bad mainboard

Try replacing the wiring harness first.

You can also test the display by holding D through powerup (you should see alternating solid color screens).

Hi Zakim

Thanks for writing to us. 

As the video is fine on an external monitor, the video card looks to be fine. 

Please run the LCD test by shutting down the system, hold down the letter D and then power on the system, after 4 secs let go the D. Observe if the lines appear during the test. 

Also do the symptoms changes (reduce or vanish) if you swing the screen back and forth? 

Do provide us your system tag#, email address and name via private message,by clicking on my name in blue and then select send a private message

Thanks to both you. For the benefit of other future users, I've posted what the test looks like.

First of all, you'll notice the horizontal lines vary their position even when I do not move the LCD panel. Secondly, the varying pattern doesn't change when I do move the LCD panel. So it indicates the cable (physical connection) might  may not be the problem. What do you guys think?