a few possibilities.
1: you have a loose wire on the stepper drive for that axis and its intermittently disconecting.
2: the drive isnt recieving a good pulse train from the PC. if its not stalling (making noise) you can try chanding to "sherline mode" in porst and pins. this extends the pulse width and sometimes makes the drives happier. ive had to do this on a few machines, incuding the nm200. this is a geckodrive issue technically.
3: if it IS, making noise when its loosing position, it can be the stepper timing as above, your speeds or acceleration could be too high, gibs too tight, or even the program if its complex contouring (mach3 has some issues with special cases).
so, a few questions.
is it making stall noises?
it it losing position in the same spot every time?