Normally stepper motors are pretty bomb-proof - no brushes so only the bearings to wear out.
Is the back-pedalling just the driven gear (backlash in the gear) or is the motor shaft also involved?
If the behaviour is deteriorating then either there is more mechanical load (exceeding the hold-torque
of the motor?) or the drive electronics is failing (many stepper motor drive chips have thermal cut-out
and will drop out for brief periods if over-driven).
[edit: actually its worth checking for slop in the motor bearings or driven-gear bearing too ]