The voltage rating of a stepper motor is usually irrelevant. The current rating is per winding, so in principle, the motor could safely draw up to 5.6 A.
However, with 1.1 Ohm windings, the motor will draw around 22 A from a 12V supply, so to what value did you set the current limit on the driver?
If the current limit is set too high, your power supply will be overloaded and may momentarily shut down, causing the symptoms you observe.