AS Tony says - make sure backlash if off and try again - BUT - the problem will return when you turn backlash back on.
Backlash on Mach3 is a seperate movement on all the axis. It appears that the computer says - move x - that is a different way to before - stop - put backlash on - stop - carry on with the move. Also in the backlash configuration, you can put in different speeds - and these seem to bear no relation to your traverse speed.
The result is, if you jog one way, then the other, the axis stops and starts four times instead of the anticipated once. Having said that it is accurate to less than a thousanth of an inch.