Recommendation of actuator
Unpredictable exciter from input to output must be as frustrating as its self-excited slip or step advance. Suggest you use a reliable servo controller or get rid of the stray noise triggered step.

Try to characterize 


the position error under different conditions, noise and level
velocity error from circuit, noise &amp; level
acceleration error, noise and level.


Did you consider effects of noise from 1.8deg steps ?