Load was not an issue in the OPs problem. He saw the same position loss when "air cutting". It is the controllers (i.e. - Mach 3s) responsibility to never command a move that violates the acceleration and velocity limits set in the Mach3 Motor Tuning dialog. In the OPs case, Mach3 WAS, due to a bug, commanding moves that violated those limits. That was a well-known, well-documented, long-standing bug in many versions of Mach3 that was finally fixed a few years ago. The bug was that under certain circumstances, when transitioning from a 2-axis (i.e. X/Y) move, to a 3-axis (i.e. X/Y/Z) helical move, the acceleration ramp on the Z axis was skipped, which commanded the Z axis to change instantly from a stop to moving at high speed, which the hardware cannot do.

Regards,
Ray L.