Erratic X

Hello Group!
I have a recurring problem when cutting a piece
of
machineable wax measuring 4"x4". After about two
inches
of cutting perfectly, my X axis goes awry and starts
moving and cutting off the wax.
The G-code is fine as I have verified it. I have
faith in my Gecko drives and I don't think they are
the
problem. I've changed my power options in the bios. I
doubt it is the driver software because it worked
perfectly until this started to happen. Why would the
cutting proceed normally and suddenly start wandering
to the left or right about halfway through the file?
All I can sat is it screws up consistently!
Any ideas from this knowledgeable group?
Bill Reidsema