CNC grief.
I know you are upset,and I hate to add to it, but you are admitting to running an out of spec computer.
One of the main computer requirements is to run a discrete video card. 32mb IIRC.
When the video calls for memory to write something to the screen and Mach3 needs to use memory, steps get missed, not all the time but sometimes. It sounds like it would be a cheap and easy fix.
I would venture a guess that an email to mach support will get you a similar answer.
Of course it could still be a complety different problem, but before you start to look for more causes, I would think that you would start with the basics, like using a computer that complies with the recomended specifications.
Mike
Warning: DIY CNC may cause extreme hair loss due to you pulling your hair out.