Have a 2000 Haas MiniMill. We run a two minute production program, milling small parts in a fixture. Then we reload the fixture and run the program again, eight hours a day. In the middle of the program, the Z axis made an uncommanded move, down exactly .100" which broke the small cutter and caused the tool holder to contact the fixture. We use a G57 offset setting for the X, Y and Z axis. This is the 3rd occurrence of this error over the last 8 to 10 months. Any ideas?