Vertical stripes on LCD display, works fine on external monitor

I'm having a problem where my video output to the integrated LCD looks like this:

However, on an externally connected monitor, the picture is fine.

I tried replacing the signal cable as suggested by the Geek Squad and, shockingly enough, that didn't fix it.

Before I throw hundreds of dollars at it, what's my best bet for the bad part...the LCD panel, or the motherboard?

Thanks!!