Skew? What is this phenomenon?

Hi, I'm using Kossel Delta printer with Repetier+Slic3er.
I was searching many forums to find out a solution of the problem.
Unfortunately, still no answer found.

This is the situation,

1) I'm having no trouble with printing small X,Y value with Large Z value object.
2) As the picture shows, experiencing failure with printing object which have relatively large X,Y with any Z value. Couldn't found any hardware issue.
3) According to the picture and my observation, There is no distortion found at Z but only X and Y.
4) There is no specific Z value that problem occur. It happened at random Z value.
5) Unlike "skew", there is no slope at the object.

Is there anyone having same issue?
I can upload more detail about my printer if you needed.

I've not got much experience with deltas, but when my Cartesian printer started to print like that it was the current to one of the motors. When I corrected the current, it sorted the problem out.

It sort of looks like a pulley slip on the stepper motor shaft. I'll even venture a guess that it is the motor driving the rods/spring that are in focus on the above picture, since the skew is in that general direction.....