I think I may now have somewhat of a theory as to why this happened: (?)
The story from the beginning: I'm printing a 5 copy print job. The first 2 copies are fine, then on the last 3 copies, there are horizontal lines. Proceeded with nozzle checks, cleanings, and test prints of my cmy and cmyk test prints. At this point, the pigment black was fine on the nozzle checks, but the colors had some horizontal streaking. Repeated above process (ya, I know, the biggest mistake). Eventually I realized that the more cleanings I did, the worse things were getting. Also, the cyan started showing up in the magenta on my test prints, which cleared up on continued printing, only to return again later after printer had been sitting idle for a while. Then the pigment black started showing missing lines on the nozzle check. Then, it nozzle checked like this: see attached.
Installed new printhead.
Then I watched some of mikling's and jtoolman's videos again. Now, I'm thinking perhaps the whole issue was somehow caused by my mismanagement of refilling the cartridges. The upper sponges especially appeared dried out or something. So, some sort of air flow issue....??? which caused a problem with the old printhead in, and then the problems followed through to the new printhead...?? Thing is though, this started with the CMY carts and now they're fine, but the pigment black is buggered.
Sure it can be a problem with air/ink flow ... also cross contamination ("cyan in magenta" - which dissappeared after some prints) could be due to the carts ... or bad sealing of cart outlet to head inlet ... or the head is sitting on a clogged or bad sucking purge pad..
I would do what The Hat suggests, as long as the carts are under suspicion.