The problem is Z will run flat out in both directions no problem. It will run feeds up to 500mm/min and 1000mm/min up to max speed without issues HOWEVER, between 600 and 900mm/min the motor stalls with a horrible scream.
This problem is typical of 'mid-band resonance'.
Some stepper motor drivers (typically Gecko http://www.geckodrive.com/support/step-motor-basics/mid-band-instability.html
) incorporate a system that more or less overcomes the mid-band problem but there is the alternative of fitting a dynamic balancing device known as a damper or dampner. These devices are quite light-weight and rattle - they incorporate small internal weights which move when the stepper hesitates (just before the stall) and in so doing changes its mass and thus prevents the stall. I have made my own dampners which I use on a couple of steppers that I usually run at around 2000 rpm and they really do work well.
Just one tip, avoid the heavy ‘puck’ type solution – it changes the mass alright but can cause other problems (don’t ask me how I know that).
Hope this helps.