cacked steppers? Z axis working erratically (5 replies)

Hi folks,
the printer, a rework using a ramps 1.4, is assembled and I'm at the fine tuning the
steppers stage. My steppers are 0.9^/2.4a per coil and they are being driven by DRV8825.

I however are having problems with the z axis. I have run the DRV8825 in full step, 1/4 and 1/32.
I have varied the DRV8825 to output between 1A to 1.8A.

Multiple problems tend to occur. The most common when homing the Z is one of the motor
will slow down compared to the other motor then catch up, leaving the X axis non-level.
Also when homing again, both motors will work at a good speed and then one motor
will stop completely.

Below is the code from my marlin build

#define HOMING_FEEDRATE {50*60, 50*60, 4*60, 0} // set the homing speeds (mm/min) normally 50*60,50*60,4*60
// default settings
#define DEFAULT_AXIS_STEPS_PER_UNIT {40,40,2000,107} // Z set to 1/4 uStep
#define DEFAULT_MAX_FEEDRATE {200, 200, 10, 25} // (mm/sec) // default 500,500,10,25
#define DEFAULT_MAX_ACCELERATION {9000,9000,50,10000}

Can anyone suggest a cause for my problems? Do I unfortunately have a couple of dodgy steppers?

many thanks,
Andrew
Oh, I bought the motors before I read the FAQs on what not to get in a stepper.