The external monitor works fine with another pc.
On VGA using the same cable?
Does your particular ProBook model use the Intel video or does it have the ATI video option?
There are a number of things you can try to help diagnose the probable location of the problem:
Is there any way you can arrange to try the HDMI output instead?
Change to a different resolution, and tell us how the lines change.
Take a screenshot instead of a photo. On VGA, if the lines aren't in the screenshot, the video chip probably has a faulty DAC, and they also should not appear on HDMI. If they show in the screenshot, it would be a video chip or memory fault, and would probably also show on HDMI, although you might be lucky and have it not appear.
If the unit has more than one DIMM and is using Intel video, you can swap their position and see if it is a memory fault, since the Intel will be using system RAM as video memory.
Edited by Platypus, 27 October 2015 - 02:51 AM.