My headset has been loosening regularly. I'm tightening it at least once a week now. And I finally figured out why. There is a washer between the top bearing cup and the lock nut which has a tab to fit into a groove in the threading on the fork. This washer is not supposed to turn (the tab in groove should prevent that), but it does. It turns when I tighten down the lock ring, and apparently it allows the lock ring to turn a bit when I'm riding so that the whole headset loosens. I couldn't figure out how that lock ring could turn without damaging the fork threads, but now I know. The washer tab has been cut by the threads. There are 4-5 threads lines cut into that 4 mm thick tab. I suppose they design the washer of softer metal than the fork so it doesn't happen the other way around (replacing the fork would be much more expensive). Now I believe I could simply replace that washer and my headset would quit loosening. But at this point I have a whole new headset (Campy Chorus) on order since my crown race has divots. This is probably one of the reasons that modern headsets are threadless.