X-Axis has issues

Hi
We just assembled the printer as by the instructions. Everything runs fine (btw, great instructions!), except the X-Axis which sometimes hangs. Mosty of the time the prints are pretty much completely waving among the X-Axis, but sometimes it almost works. This is the best we could do so far: (it should be a cube)

We checked the alignment and adjusted it as best as we could (roughly a tenth or two of mm accurate now). The axis are oiled, we have the latest firmware installed. But it does not help.
It seems that the Axis hangs at some points for a short moment. I am pretty sure that the motor vibrates when it hangs. No Belts are flipping.

Currently, we are pretty clueless what to do next.

Layer shifting is almost always a mechanical issue. Check the alignment procedure on the [color=#408040]Wiki[/color].

If everything is OK then check the reference voltage of the stepper drivers (procedure on the [color=#408040]Wiki[/color]).