Stepper motor current specifications - 12V/350mA, but draws more at lower speeds?
When driving a stepper motor, the maximum torque is proportional to current, and it is your job to regulate the current. What you're doing is providing a fixed-voltage drive to the motor, with unregulated current. With the motor commanded to a stop, only the resistance of the windings will determine the current.

When the motor is not stationary, as the rotational speed increases, given a fixed voltage the currents will decrease. This happens since the motor windings are inductances: the currents will decrease the faster you switch them. 

The only workaround is to use a higher drive voltage with a current-mode controller. In most cases you don't want to exceed 48V drive voltage so as not to break down the insulation on the motor's windings. The current will be limited in all cases by your current-mode controller. A current mode controller can be a simple linear current source, or it can be a more efficient switched-mode current source.