There are a few things that could cause this, and it was a good idea to connect it to your TV, doing that has eliminated a fault with the integrated video. Like you say, it could be the display cable. The backlight, or the inverter could be likely suspects too. Which model Presario is it?
If it was solely down to the internal display cable, which is typically fed near the hinge mechanism, then you might have seen the display flicker as you moved it or opened and closed it. Did you notice this?
I've had a few laptops over the years that have this same problem. In two cases, its been the ribbon cable inside that has been the issue due to the constant crimping of the cable inside. good work on the troubleshooting.
It looks like a case of having someone open it up and checking the path and condition of the video cable and reseating the connector.
If feel you want to do this yourself, you can download the service manual from here which will explain how to disassemble your laptop to the point of checking the cable and connector. Have a look at page 56, it illustrates the video cable routed near the hinge.