Z-stage stuttering

Z-stage stuttering
My little cnc almost runs perfectly. I have been trying a few gentle milling an drilling jobs and it has become apparent that the z-stage loses position after only a few movements. Often it would turn the wrong way when a Z positive command was sent.

It will happily go down regardless of the previous direction but sometime an up command would stutter and whine if the previous direction was down. It may be easier to see in the video.
 I spent quite a while checking the calibration and acceleration to no avail. Long story short, it was a driver problem again. When wired into an arduino and a different stepper a change in direction one way sent the motor a few degrees in the previous direction first. I hadn't found this problem on any forum so hopefully this is of use to someone else.