The first mistake is using the holding torque as a measure of performance to specify the step motor. Holding torque defines the torque a motor produces when maintaining a position and not moving. It is generally a poor indicator of the torque the engine produces when moving.  

When a stepper motor starts moving, the produced torque falls precipitously from the holding torque value, even after just a few rpms. As speed increases, the torque falls further. For this reason, do not select a stepper motor based on holding torque alone. Instead, refer to published speed-torque curves.

Shown here are step motors from Applied Motion Products with various stack lengths.

The second mistake is failing to understand the nature of speed-torque curves. A speed-torque curve represents the torque at which the engine stalls. When a step motor stalls, the rotor loses synchronization with the stator and the shaft stops turning.