Weird problem - stepped/staged deceleration on Huanyang 2.2kw Spindle

I have the Chinebo 2.2kw water-cooled Huanyang VFD/Spindle setup. It's been put away for some time and brand new. I originally set it up some time ago as per the instructions on the sticky thread on this site. This weekend I have it mounted to my machine and fired it up and whilst it runs fine it does a very strange thing when I power off the spindle - it decelerates in about 4 or 5 staged steps - if that makes any sense - if you imagine the tone of the spindle as it shuts off there is a definite stepped deceleration tone "So, Fa, Me, Re, Do" tone as opposed to a smooth deceleration (no glissando for the musically educated).

I have tried PD015 at both 0.8 and 1 and it makes little or no difference. Has anybody encountered this before?

I am beginning to wonder if my mounting clamp is too tight - it required a strong arm to close the bolts down so maybe?