What do you suppose is causing this issue?

If that's the clearest shot you can get of it, I'd venture a guess towards your encoder strip, with a slight possibility you need a good overhaul.

 

Those faint lines are indicative that you may have a tiny bit of dust or debris hanging around the print head that is building up with ink and redepositing it onto the media as the print head moves back and forth. A thorough inspection and cleaning under the entire print head carriage with a flashlight would be the first course of action that I would take to rule this out.

 

could be a blurry photo, but looks like your magenta is out of alignment as well.

 

Is this on the Gerber?

 

If it were a Roland (know nothing about Mimaki) I'd say clean the encoder strip from end to end.

 

Is the head as ancient as your printer? Could be the head going out and you might be getting droplets building up on the head and dragging. Can you see ink starting to drip out of the head?

 

If ink is dripping out of your head I would say its time for a new head. I never touch the head on our Mutoh just clean around it.

 

aye that just looks like contamination of sorts.
$5 says you've "cleaned" the capping station & replaced the wiper BUT inside of the wiper station you've got some gummy bears wiping their butts on your wiper blade.

 

This happens mostly due to VSD settings.Smaller dot size tend to spary fine ink and you can see this spray in white areas.Like you print some word and you will see ink spray in printed words.Remedy is use larger dot size.Hope this will fix problem

 

The only time I have ever seen ink droplets is when printing banner with a lot of scrim fuzz on the edges,
they "tickle the Nozzles" and cause them to drip (I know that frase sounds bad) If this is the case, just take a lighter
and burn away the fuzz.

The others look like your material is "tunneling" as it's going through your printer, causing a head rub and
ink-cross-contamination... but that's speculation, have you sat there and watched it cause an issue?
If not, you might want to set up a camera on your printer to see what it's doing.

 

You might consider replacing the damper for the black. Over time, the solvent ink can impact the O ring that seals the damper to the print head manifold and you lose the sealed integrity of the ink line system. This can cause slow leakage from the print head causing random ink drops, mostly where the carriage changes direction. As the black and magenta inks are shared on the same head, this seepage can also cause black to be introduced into the magenta as it prints. You may need a print head, but the damper is a quick inexpensive replacement and the prints seem to indicate ink leakage.