mach4 not close the loop ,so there almost not any way to know that mach loose the real position,i use servo motors and in that case the servo pack close the first loop ,but also not the second loop ,so also in mine i had that problem
as my experince i can sugest you cheack ground loop
first be sure you have one "0" for all yours power supply ,if you have more then one conect all "-" togethere and ground them