Machine is Mach 3, Pokeyes57, Windows, Servo Motors

My problem: I have had frequent issues with my machine (cnc router) randomly moving in the middle of a run.  Most recently I was on line 77000 of my run when the machine just drove in the negative Z direction until the spindle hit my part (shutting down on collision).  When I look at the G-code just before / during this event there is nothing that would drive this behavior.  I have had these issues from time to time where the machine just takes off.  When I reboot the PC and startup the controller/ Mach 3 software it seems to do fine for a while, but in long runs or if the machine was setup without a restart, I get these 'ghost' movements.  How the heck do I trouble shoot this issue?

Perhaps there is a buffer or memory issue with the mach 3 / PC / Pokeys57.  Any ideas, have otheres seen this issue?