Hi,
that sounds like a mid-band resonance. Two phase steppers are very prone to a resonance at a moderate speed, if you can get them past the 'hump'
then they will go a lot faster without losing steps.
What microstepping regime have you in use? One of the real advantages of microstepping is the marked reduction in mid-band resonance. I would
recommend either 8 or 10 microsteps per fullstep, that is 1600 or 2000 pulse per rev. You'll get most, if not quite all, the benefits of microstepping
WITHOUT requiring really high pulse output rates from your controller.
Craig