HP Z6810 printing vertical lines every 1/4” any ideas?

[WideFormatTech]

I am working on an HP Z6810 dye based printer. It is doing something I have not seen before. It was printing faint vertical lines using all colors (seen under magnification loupe) every 1/4” apart in all prints, including internal calibrations and test prints. It was running F/W version PX6_03 and I did reset EEROM. Then I found newer firmware on HP website PX8_05_02_00.2 which I installed. Now it prints these vertical lines every 1/8”. See below, newer firmware on left, older firmware on right and second picture.

 

[greysquirrel]

I would run your printhead diagnostic plot to see if any heads need replaced. looks like black. Id start there...then it I'd look at trailing cable.