Good Morning Okaytrythis,
Sorry for the late response, it's been a busy week.
Just to clarify, the connector cable i'm referring to that may be bad would be within the notebook itself. Typically there's a cable which runs inside the hinge of your notebook to feed the display on the top half the video feed from the components on the bottom half.
Sometimes this gets worn out from hinge use over time.
So far, this might be a good thing that the other monitor doesn't have issues as this may be a sign the display is bad and not the motherboard itself. Unfortunately, if it's not in warranty the cost of repair might run you up a couple hundred bucks.
Here's a link to a diagram showing the display and cable, Number 2.
http/www.laptoppartsexpert.com/images/cms/compaq-hp-pavilion-laptop-parts-exploded-numbered-cmp.jpg
Sample part page with pricing for a replacement display: (I can't vouch for this site, I've never bought from them before. Shop around, this gives you a rough idea of the cost of parts alone as a starting point.)
http/www.laptoppartsexpert.com/i-2877103-lcd-screen-display-14-1-wxga-tft-antiglare.html
So, based on your findings that the external monitor does (((Not))) have the same issue as the display on the notebook itself, I'm going with the most likely culprit being the display of the notebook itself is bad.
I'd be open to anyone else's opinion regarding the issue! If this was my notebook, this is the direction I would take.
You may want to contact support by phone just to get the final word on what they can do for you to assess your options. If you feel comfortable taking on a repair like this on your own, we can likely dig up a service manual to help you out.
Hope this helps you out, cheers!