Gotcha. If it was me, I'd start with two things. First would be pulling the headstock apart and addressing the bearing adjustment problem. The inner race on the outer bearing needs to be able to slide on the spindle to set the preload. I did mine the old fashioned way like you'd polish engine crankshaft journals. If you haven't done that it done by bolting the spindle down firmly, wrapping sand paper around the spindle, wrapping a long shoelace or the like around the paper a few times and pulling the ends of the laces to turn the paper. There are probably YouTube videos that show what I'm describing. It works better than just trying to hand sand as it keeps pretty even pressure all the way around the part. If you decide to do it use fine grit paper, like no courser than 800 to start, as yours is probably pretty close. I may even start with 1,000 as yours is probably within fractions of a thousandth if the bearing already goes on but doesn't slide. Go slow and check fit often. As we know It's easier to remove material than put it back. :-) ?You just want it slide, just. If you go too far then the spindle can move in the inner race and we don't want that. It's a time consuming process, lots of cleaning and testing for maybe only two or three rotations of the paper before cleaning and testing again but it's worth the time. You'll be able to set it perfectly.?

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My second thing to check would be the lead screw and half nuts to see if there's a burr or anything causing irregular movement and adjust of the base, compound and cross slides. That may cause the ghost thread. You can check if it's from the compound or cross slides by locking them down tight with the gib adjustment screws and taking a test cut. I'm sure you know it's really a process of eliminating variables until you find which part is causing the problem and then addressing that part to eliminate the issue.