It could be a motor stall problem. Try reducing the acceleration, you can always set it back up if no difference. Your Mach3 steps per looks wrong. I am very tired at the moment, so if anyone would care to check my calcs please let me know if I am out.

Motor has 15 tooth pulley, Leadscrew has 30 tooth pulley (I would have expected this to be the other way round - but no matter)

2:1 pulley reduction
2mm pitch leadscrew means axis moves 1 mm per revolution of the stepper, ie 200 steps, 8 microsteps means 1600 pulses per rev, i.e. mm.

So steps per in Mach3 should be 1600 not 2000.

Have you done any dimension tests yet or has your current problem overshadowed things ?