Artefact on X axis

I had redo all check multiple time. I did all combination of slicer and front end.
Now, I'm sure that
- the X axis stepper is 0.401.
- the nozzle is 0.3
- the printer is levelled
- the X axis is levelled
- the Y axis is levelled

For the picture I use Repetier-host and Skeinforge.
The gcode file seems ok in the Repeteir-host view.

I don't understand why the printer start an edge 0.5mm. In some pieces I printed, this leads to a gap between the edges and the fills.
I have better result is I rotate my pieces of 45 degrees.



Could you help me?

This could be backlash on your X axis - make sure your belt is tight and the grub screw in your pulley is not loose.