hi,

i had same problem as you. the fix for this: The problem not with your setting in mach3 or the driver board.

The problem is the wiring. your wiring need be fix.

1. the wiring a attach to the power supply with tire wraps. That's A NO NO. remove it from power.
2. In that box, You need use 4 wire shielded cable for each stepper motor, from contoller board to the 4 pin connectors.
3. You need to put EMI AC filter. remove the one that come this box it's cheap chinese make. You can one on amazon $10 USA made.
4. You need to use 4 wire shielded with GND wire. 4+GND, For DC power. 2x red/2x blk plus shielded wire for GND.
Don't get confuse with the AC Ground wire and DC Ground, They must be separated.

5. And Most important (REMOVE) VFD from Box, This VFD is **Shielded** attach to outside with plastic mounts. No metal contact between VFD and Box Controller.

6. The stepper motor cable's from Box to stepper motor MUST be shielded cable 4pin wire + shielded GND.

I did this my 6040 and has been work 100%

Good Luck.