You should be able to remove any guards etc. and take a look at the couplers. Check that there is no play at the coupler and you should be able to check the screws and make sure they are tight. Are you using Mach3? If so have you used the axis calibration utility to check and adjust the axis travel? You can find it on the Settings tab just above the red Reset button. It will help you determine if the axis are moving the same distance as what is input and make adjustments if needed. Does it seem to accumulate in the same direction each time? Which axis are affected by this problem? Make sure all the mounts for the ballscrews are tight and not lose. Also make sure there is no play at the spindle or the Z axis head.