"Perhaps you need a spacer. I've seen this problem many times and often that's all that's needed."
I think that's a good idea, I imagine the threads in that region have at least seen a fair amount of drama now after having tightened and loosened so many times, on top of whatever wear on the steerer threads had already been there to cause the problem to begin with. For whatever reason, I have never seen lock-tite on steerer threads; and therefore generally just feel uneasy about it. Good luck, man.