Very Fine Horizontal Banding Epson 7600

My Epson 7600 suddenly began printing very fine horizontal bands using a K7 Special Edition ink set to which I have converted the No. 1 slot to HD Ultra Matte Black . It had been setting for several months and required extensive power cleaning and print head cleaning before it would print a decent nozzle check. I say “decent” because the No. 2 slot, Shade 6, is still only printing a partial nozzle check. I printed several images successfully before the one that demonstrated very fine horizontal bands.
What could be causing this problem? Thank you.