The stroke/white banding direction is parallel to the head carriage movement I guess. Demand for the heavier black area there seems to be handicapped by the black ink supply. This could be a shortage of black ink in the ink channel for whatever reason, cart near empty, filter in the head blocked, cart pumping not working correctly etc. It could be a head that is not meeting demand in its nozzles pumping capacity due to age for example or a bad sensor on the black ink buffer there. Last but not least a glitch in the electronics at another point like the head carriage board, the ribbon cable to the head carriage not 100% anymore. I would listen whether the cart pump is activated often with a small job of black printing, take out the cart to check its weight against a really empty cart, check the membrane pump on the cart, any leaks on the cart. Take the black head out, clean the nozzle surface, check the contacts on the cart and in the cart slot on the carriage, make sure they are clean and dry. Align heads if the black one is accepted at all and do a proof again with a heavy black area. If it still does not work get a new head. If it still does not work check all cables and reinstall the driver etc. If that does not work it is something between the printer network cable and the head connection, could be the ribbon cable to the head carriage having a bad connection, broken wire there.
Met vriendelijke groet, Ernsthttp://www.pigment-print.com/spectralplots/spectrumviz_1.htm
December 2012, 500+ inkjet media white spectral plots.