In my case, just like Gerry's reply, I was missing steps and pulled my hair out trying to find the culprit. Machine 2.5D all day,but would randomly skip steps on 3D stuff and you coud not hear when or where it happened. Playing with settings didn't do any good.
So I replaced the steppers with ones of two and half times the original torque rating, got rid of the backlash in X and Y axis by replacing the bearings and adjusting the ball screw nuts, and life is good again. Now if i would only get off my lazy butt and redo the Z axis life would be really good on the mill.
RICH