answer:Can you unobtrusively fasten the jewelry piece to the chain so that it doesn’t slide on the chain at all? Failing that, which may not be desirable for several reasons, perhaps you can add unobtrusive stops in the chain that prevent the jewelry piece from moving “too much”. I think the clasp tends to move toward the lowest center of gravity – which the jewelry piece itself also wants to occupy, of course – so whenever the chain slides backwards or forwards, the clasp is going to be biased towards “wherever is closer to the center of the Earth”. And so it goes, and keeps going. My point is that if you can get the jewelry piece to always or nearly always occupy the favored spot in the center of the chain, then it’s a lot less likely for the clasp to move, because it has to fully overcome the inertia of the jewelry piece at the center.