If printscreen didn't record it, it would be hardware related - not software related. A printscreen only records what is 'sent' to the screen; not what is 'on' the screen like a photo. Due to the fact that the lines didn't show up on the VGA/external screen - it rules out the graphics card. It seems that the laptop has stuck/dead pixels on the LCD.
I don't see how a stress test on a CPU and GPU could help in this case; unless you were testing stability of the machine (fixing the crashes, another problem altogether).