Robin2:
A very wild guess (very wild) is that you don't have good GND connection to the Arduino
I don't think that's a wild guess at all: quite reasonable based on the OP's explanation.
I had a similar experience 20 odd years back using a 298 driven off a 297 to control a stepper from a PC parallel printer port.
Noticed that when my bare foot touched the floor, the motor ran. That was completing the ground (literally) from the PC case to the metal box where I had all the bits and pieces, and which was in my hand, thus acting for the wire I had left out.
But OP, for the 293 to work you must have had a ground?