No experience with the Z6100 but if it is that critical it could well be a slight misalignment of the heads. Do a heads realignment but first clean the rod the head carriage runs on and oil it with the prescribed oil. That head has to run fast. I have done the same on a Z3100 and Z3200 when I noticed a small vertical misalignment.
met vriendelijke groeten, Ernst Dinkla
Try:
http://groups.yahoo.com/group/Wide_Inkjet_Printers/