Wiring looks fine, and the current is set to 4.2 A so that should be ok also.
The problem that you are seeing is a typical noise or low signal problem, or possibly both.
These are the points you need to address:

1) Use Shielded cable for all signal cables
2) Shield the motor phase cables.
3) keep signal and power / phase cables apart, and if they need to cross, they should cross perpendicular.


Also check the signals from the pc, the voltage should be about 5V, it it is low the driver can miss pulses and you can get the stuttering you are seeing.