I am still curious as to why the problem only shows itself the slower I try to step the motors.
I guess I missed that in my first reading of the problem. I see now that the number you are decreasing is a divisor for the step time.
If my calculations are correct, a value of 20 gives you just over 3 milliseconds per step. I suspect you might be running into some harmonic frequency where your step rate is fighting against the natural oscillations of the rotor.
Increased voltage/current may
help to control that. What current is the motor pulling at the voltage you have now? You might also have better luck with INTERLEAVE or MICROSTEP at those speeds.