powerSTEP01 motor current unstable in one direction

[CTapp.1]

I have a powerSTEP01 driving a stepper motor with the coil current set to 2A. However, the current flowing in the coils is unstable when it is flowing in one direction:

Sorry about the image quality - I have an old 'scope, so had to use a photo ;)

The image shows the coil current as the motor steps. The current is stable at 2A when the coil is energised in one direction, but not in the other.

Any ideas what can cause this?

[CTapp.1]

Looks like this was an instrumentation issue - doesn't show with a new current probe!