This keeps getting more interesting all the time. The pins used on the ss to drive the buggy z axis work fine on one of the other axes.
I diagnosed the drive board in question thoroughly, there are no shorts, opens, or other problems, the optoisolator currents for step and direction are 8mA and 7.5mW respectively, almost exactly what I had calculated they would be. No power supply shorts, no other interferences on th ez drive board I could find.
So I removed the smoothstepper from the installation, reverted back to mach3 alone, and it works fine. As it always did. The z drive simply opened its resistor for some reason but is now perfect again. But the smooth stepper doesn't work at all. It constantly trips out on emergency stop all the time randomly, and won't drive that z axis. I don't understand it at all.