All stepper motors suffer from a 'strange' phenomena, particularly al low speeds, that can cause a dramatic lost of torque or even a stall of the motor itself. This phenomenon regards the 'resonance' of the motor, a particular frequency where the motor starts a desyncronization because its natural oscillating frequency has been reached.
The choice of a stepper motor it is not always an easy matter. Usually the question is: why should I spend 80-150 US$ for a good nema23 stepper motor when I can find the same motor in a surplus market for 5 (or even less) US$?