error in DOS not in cncpro

Anyone ever hear the term "number chain"? A machinist used this
phrase when discussing a mysterious move I have in a Gcode file
run with cncpro.
Problem appeared in Z axis when I ran a file to engrave a sign on
my router. As program is running I will get a -Z move of aprox. .5in.
It is not in the gcode, extensive testing of drivers,mechanicals
proved it is not lost steps,coupler slip etc.
I created a few new files from similar but not same drawing and no
error. Went back and ran original file at slower speed SAME error!
I hooked up to a new PC,fresh load of cncpro ran file SAME error
created yet more new files ,no error.
My machinist friend then told me that DOS could be causing the
problem due to a "number chain error".
Honest,I am not making this up! My router has been rock solid
reliable.
If anyone has a machine that with travels of X24,Y16 Z4 or larger and
is using CncPro I would like to email this Gcode file to see if you
get the same error. File is approx 5000 lines.
email me offlist
Carl