I'm sure the repair shop tho't of doing this, but the first thing I would think to do is re-calibrate the screen. Probably has already been done, but you know the instructions are in your manual, and can never hurt to ck that out. It is very puzzling that only the zz stitch is acting up. I think I would dismiss the problem if there is a substitute stitch you can use instead of the zz--until the machine needs some other repair in the future. Then throw in this additional complaint for them to work on while it's in shop. Just me.