matth
How long have you had your drives, as I have not seen this problem before, it must of been that batch of drives you have
What settings do you have in Mach 3 for motor tuning Velocity Acceleration, If your Velocity is not set correct you will trip the drives with over speed, if you have this set correct you won't get any Drive tripping
If your Ballscrew is 5mm Pitch then the max Velocity setting you can have for the 400w motor is 600