@CaptainSnowball As both motors are connected to the Z driver pins, I'd guess this was a motor-side problem. Your Z axis motor settings look sensible, shouldn't be causing skipping, though does this happen at all speeds, or the fastest speed the axis will allow? Because the Z motors are wired in series, any break or poor connection on one motor would cause the same effect on the other side. Wiring is series keeps the current the same, but halves the voltage, though the stepper driver should take account of this and increase the voltage (if possible). As you have 1.5A rated steppers, you can set the current to 1200mA.
Most likely something is stalling the motor; either the axis is jamming (which is the most likely) or there's a problem internally with the motor, for example metal swarf in motor - test by disconnecting both Z motors, and turning the motor shaft with your fingers, feeling for any point that feels tight or the motor feels rough, compared to the other motor.
Are the motors the same (brand/type)? Unmatched motors could have different resistance, and one motor need more current. Even motors that are the same manufacturer and part number can vary. Are you using 12V or 24V PSU? If 12V, it's possible (depending on the specification of your motors) that it's not enough to voltage is going through both motors, in which case wiring each motor to a separate driver, as @Veti suggests, but this does mean you need to have a way of levelling your X gantry, ideally with a probe. See https://duet3d.dozuki.com/Wiki/Bed_levelling_using_multiple_independent_Z_motors
Ian