What speed were you running the steppers at? if your steps per unit are set too high that may be problem as well.
At the highest RPM the stepper's torque rapidly decreases.
The 180 oz/inch steppers should work fine on a slick machine like yours unless the RPM is set too high coupled with the steppers not getting enough current. I have read somewhere that the steppers will pull a need more current than what is shown on their label if its a per phase rating .
I have the 127 oz/inch motors from Hobbycnc rated at 2 amps, with a 12 amp transformer and only had them stall when I had the machine running at 60" min. My JGRO obviously has a lot more binding pressures (backlash nut, slight misalignment of lead screw, no ballscrews) than your K2.