my printer was doing exactly the same, I was going crazy about it and I almost buy a new print head, (with labor about $1,300.00), I start using Jetbest ink and oracal profile 3561 and............. magic.......... the print heads after 1 or 2 days of normal printing (2.5 hours a day for me) cleaned itself, now when I do a test print it looks pretty and the black is black, not stripped like your sample.
try to flush the head with solvent cleaning fluid from the dampers: remove the ink line going to the damper, place a hose on the damper and at the other end a syringe with cleaning fluid and push it through, (I did and remove a hose from the pump and placed to the damper) just make sure the cleaning fluid falls in a container.
good luck
guillermo