From what I've read over the years, it seems that some people run into this all the time, and most never see it.
Of course, that doesn't help at all.
One thing about version 3.043.022 mentioned earlier. It has a serious bug that can cause Mach3 to hang on M6's, depending on the contents of the M6Start macro.
While testing when I first found it, I could get it to freeze when the M6Start had just one line of code. It definitely doesn't work with the M6Start in the 2010 Screenset.