Narrow white vertical lines or "scratches" on black printouts.

[cdysthe6]

This printer has worked flawlessly for 2 years. Then when we came back from a two week vacation there are white lines or "scratches" on all black printouts. It actually looks like the black ink is being scratched by something but that may not be the case. All colors print perfectly. We tried to swap out the black ink cartridge (HP original) but the problem remains. We have tried printhead cleaning several times but it makes no difference. Black printouts are of bad uneven quality in general, but the white stripes or "scratches" in black printouts is the worst. Gray is also uneven.  You can see the problem here: 

I am not able to get the image embedded here so please click the link:

                      https://i.imgur.com/3Pf3pjT.jpg

What could the problem be?

[Repairatrooper · Accepted solution]

Black ink fade, pigment has separated from the ink carrier and blocks some of the nozzles. This was a common issue and inks with an expiry date earlier than 2021 may contain an outdate ink formulation. The fix is to ensure you have cartridges containing the new ink and then to print 100 entirely black pages on the printer. Make sure your firmware is up to date before you start.