So I seem to remember someone mentioning this before but can't find the thread. I ran a job earlier today, came out fine. Running another copy, and the beast is doing head checks every eight passes. It's taking forever to print. Is it just annoyed with me for letting the PK cart get low (printing on matte today, so who cares), or is there some deeper issue?
This appears to be a systemic quirk of the Z3100. After printing with it for a year, I have noted this repetitive back & forth head waggle under a number of circumstances.
It definitely happens when one of the carts is low; I suspect the problem is that the cart's ink level indicator isn't very accurate, so some may be almost empty and the printer struggles to deal with nozzle starvation. It's worth replacing a low cart the first time this happens, just to maintain your sanity.
It starts happening more frequently when the heads are getting gunked up. The Z3100 tends to overspray, with ink gradually collecting on the bottom of the head carriage assembly, especially near the matte black head. A thick bead of ink will also start collecting on the left-hand "diaper pads". In my experience the answer is to pull all the heads, clean them off with a Q-tip and distilled water, scrub the bottom of the head carriage with a damped coffee filter, and scrape the excess ink off the top of the diaper. Then reinstall the heads and realign them, and all is well. This doesn't happen very often; it took 7 months of printing before I had to clean up.
Finally, it occasionally seems to occur out of the blue, even if the carts are okay and everything looks clean. One or two cycles of head cleaning seems to solve the problem.
Hope this helps.