Z-Axis on i3 clone won't move at all, Melzi A4982 driver problem
I've replaced the Z-axis stepper driver with an off-board Pololu-style A4988 driver by soldering leads to pins 2 and 3 on the Melzi board, then connecting those to the driver on a separate board. The Z-axis now moves correctly. When I encountered issues with the step pin (pin 3), I connected it to pin 12 (bed heater) instead, leaving the bed to be heated manually. This has worked so far and the only problems I've encountered were minor issues with electrical noise, which I was able to fix with a shielded cable.