You did not say what paper you are printing on. Sounds like silver gelatin? I stop printing on silver gelatin, because all stochastic imagesetter negatives depending on the image, one can see the pattern. I have been only printing kallitypes. I also found rastus to do a better job than diffusion dither or icefields. I also process 1800dpi bitmaps at 1800dpi due to mechanical limitations of the imagesetter at 3600dpi (the problem does not show up all the time, but it does).