Cut your Z axis acceleration in half.
I had been running my machine for several months, and it had never lost position a single time. The first time I did a detailed 3D carving, within 10 minutes, the Z axis was above the part.
Lowered the Z accel by half, restarted the job, and it ran flawlessly for 2 hours until the job was done.
Bottom line, is that all jobs are different, and some put a higher load on the system.
And btw, my Z velocity is 30, and I lowered the accel from 10 to 5.