I put together a cnc mill a few years ago and messed with it as I learned about this stuff, and this past year I decided to do some upgrades to the electronics.

The main things I upgraded was a motion control card and separate drivers for the stepper motors, was using a Gecko G540 all in one unit.

I've had these 495oz steppers, rated at 3 amp current & 5.1 voltage, and they seemed to work fine so I just kept them.
NEMA23 495 oz/in Stepper Motor KL23H2100-30-4BM (1/4?Dual shaft with a flat) | Automation Technology Inc

And I got the ATI's KL-4042D digital drivers, rated at 36v & up to 4.2 amps output.
KL-4042D Digital Bipolar Stepper Motor Driver-32 bit DSP Based | Automation Technology Inc


The problem I'm having is the steppers won't turn under load, they make a clicking noise like they are trying to and I can see the shaft jerking, (example) kind of like a stripped gear type of noise.
Tried setting the amps higher but that didn't work.
Then I turned the power supply up from 36.5v until the warning light came on on the driver, then turned it back until it went out, the light went out at 43v and I set it to 42v, but the steppers are still doing the same thing.

Did I get too small of a driver (voltage wise)?
Or could this be a "settings issue" I'm missing in Mach3? I did try messing with the "motor tuning" in Mach a little but it still did the same thing.

Any advise would be helpful