You say "only with SS and...", but does that include aluminum, as well? Could be flashback. if you haven't done aluminum yet (since that would do the same). At that kind of power, a melted pool of metal makes a really nice reflector. Also, make sure your beam is aligned correctly. If it's off by the time it gets to the focusing lens, it could be heating the lens carrier and causing more havoc. Are you sure the air assist is getting to the focusing lens?