we've encountering a printing quality issue with a DC 242 with Fiery RIP (and also same problem with a XC 700). We noticed that using any type of dithering, on every prints and regardless of which type of image I'm printing or type of paper I'm using, there's a sort of vertical lines "background". Is it due to a mechanical reason or it could be a file problem? I've also attached an image to explain what I mean. In the image given you can notice it espacially beside woman's face.
the drum gears are slipping. I've had this on my Xerox 770 quite a lot, which is the same engine (revamped) as the 242/250/252 line. you'll need the have the techs get into the drum drive gears in the back to find the bad set.
Hope that helps.
Run some half tones of each color. It looks like it is in black. It could be the drum drives or developer housing.