I've been able to reproduce the issue consistently. I recorded the operation starting at the beginning of the code recording the sequence of moves and each time the Z failed to move. Then I re-ran the code and found that the Z failed to move at the same instructions during the second run. I was also able to identify that the failure is not a dependency of the previous XY location prior to the failure. This tells me that it is an issue with either MACH3 or with the communications to the control box. At one point I received an ESS buffering error which leads me to believe that there is an issue with the ESS driver that communicates with the control box or the ESS board itself.