Hey guys,
Thanks for all of your help and suggestions. I returned the defective refurbished camera back to the dealer, and bought a new one from Costco (it was on sale with the 18-55mm and 55-200mm VR for $599!). The new camera exhibits none of the dreaded black lines I had with my previous camera, so I'm happy!
I've contacted Nikon showing them that my new D3300 did not have any of the black lines that they were so adamant to claim as "normal operation". Here is part of their response:
Also consider that this is an entry level camera. The sensors used with lower end cameras will not typically bear the same scrutiny as professional model cameras, nor should they, that is not their design purpose. Professional photographers are more discriminating when it comes to image quality, and of course, higher quality sensors are used in pro models as well and generally provide a greater signal to noise ratio. This is also a mass produced camera, so, even with strict quality control, it is possible to oversee an issue or have a problem emerge over time.
They are still denying that there is any problem with the camera I've returned, AND they are in the same breath essentially saying that the D3300 is held to a lower standard because it is an entry level camera. Strange considering these two statements seem contradictory.