when this happens have you tried going back to you home position and if you do does it do it?
i had this once with mach3, it would be fine for hours, but as you say it would then go off on one. when it did this the DRO's was still spot on i.e i wasn't losing position. if i sent it back home it would go there. i could then restart the g-gode and it would re do the part fine.
when this was happening i was using mach3 lockdown R3.043.022. i then changed to R3.043.035. its been fine ever since.
i'm not saying this will defiantly solve your problems, but it worked for me.