I do not think it is anything to do with the taper but more likely just an issue with Mach threading.
If I recall this was a reported issue on several occasions and I am sure Art reckoned it was to do with spindle slowing down during the threading cut and screwing things up, I could be wrong about that however. There was a lot of work done on threading at one point and things were changed significantly, don't recall the exact version things were altered but if you are using an older version of Mach or your m1076 is old then it may be worth updating things, just don't update to ver 066 as it is screwed, especially for Turn.
Hood