I had similar results at one time, I think you may need to drop the velosity down a bit in the motor tuning.
Start off low and increase until it starts occur, then back it off a little bit. If it is still happening no matter how low you set the velosity and accel then we need to look at something else.
You must restart mach3 when setting slave axis, but you are probably aware of this!
also I've found that if I power up motor drives before the PC, I have had slave driving oposite to what it is supposed to.