Hi Boothie,
Sounds like you need to retune to slightly lower settings than you currently have. Typically the tuning process is to tune upwards till you have lost position and steps then drop the tuning back until things a completely stable. I am currently going through the process right now and am having the same problems, but right now mine is only 1 axis that is giving problems. I have Mach2 installed and very stable so I can continue to work while taking my time on the tuning of Mach3.
Mike
No greater love can a man have than this, that he give his life for a friend.