while a replacement under warranty certainly is generous, to the OP: 1500mA at 24V is pretty much the highest (right, or is it 2A max?) you are to push the built in drivers, so feel fortunate that dc42 is being very cool about this. I run 24V on a delta bot and my currents are all around 800mA (obviously, the values are motor-dependent, but still, (I am inclined to say) it's unnecessary under normal circumstances to ever run the drivers at their upper limit…lest you get results such as the ones you've gotten. Just a thought for the future!
Have a nice day. Also, the intermittent direction changes in your Z stepper point to wiring problems for that motor. Before popping that driver too, please re-do the wiring and ensure you've got the correct pairs going to the correct locations as well as no intermittent shorting. Just in case you didn't know this: ensure your power is completely off prior to connecting or disconnecting stepper motor wires, as often doing so while powered up leads to blowing the associated driver, prob. the majority of the time (which is akin to an intermittent short).
HTH.
--jonathan