clunks / missing steps - DM856 and single pulse operation

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Hi everyone,

hoping someone can suggest things to try as I'm a bit lost as to why this is happening.

I have a DM856 driver and a small Nema 23 motor, all connected to a SPS705 68VDC power supply (I was told the voltage on the PSU is a bit high for this motor, but it should run ok). This is driven via the PUL / DIR input from a custom built uC based board, which acts like a jog / test board, you press the '+ / -' buttons to turn the motor one step CW or one step CCW. If I press a button and hold it down, the pulses are generated in a cycle and the motor moves smoothly, but if I only press the button once, every now and then the motor 'clunks' or jumps a step, sometimes going reverse etc. Here's what the pulse looks like:



The first two (from the left) screenshots show the pulse generated by the uC / electronics, pulses are 41us wide and as per the manual the DIR signal is in front of the PUL signal (DIR is not shown here). The screenshot on the right is a pulse from a pulse generator, not much different from the micro controller generated ones. With the pulse generator the motor runs smoothly, even at 1Hz :O

Anyone got any ideas as to what might be causing the 'clunks'? the motor isn't loaded, it simply sits on a desk with nothing attached to it.

Regards,
T.