You are running 4.2 Amp motors on a 3.5A limited driver. This cuts the 282 oz motor down to only 234 OZ. I don't recommend this motor for G540. It STILL should run pretty good though. 234 oz is plenty for a Taig.
What size current limit resistors are you using?
How do you have the motor wired? Bipolar Series or Bipolar Parallel?
Have you tried setting the trim:
ADJUST: This trimpot adjusts the motor for the smoothest possible low-speed operation. Set the motor speed to
about 1/2 revolution per second and then turn the trimpot until a distinct null is noted in the motor’s vibration. This
will result in the most even microstep placement for a given motor and power supply voltage. The default setting for
this trimpot is at ½ -turn and the setting for your motor will be within +/- ¼-turn of the default setting