SSS renders black on some frames

from this thread: http://blenderartists.org/forum/showthread.php?t=159439

here’s what’s happening: http://www.vimeo.com/5454968

don’t know what’s going on here; the SSS settings on the second fish are the exact same as the first one. i tried turning off the mist, tried rendering only the fish (no hook or background), but no dice. anyone else experience this?