Random extreme slow downs on OS X

Without warning, it slows down to a fraction (1/8?) of the prior fps rate and becomes unplayable. It also slows down the entire machine, system and other apps. Yet the Activity Monitor is showing only about 50% of CPU for SotA. Happened three times so far, once in Rift's End, once in Central Brittany, and once before - I think in Veritas Sanctuary or Aerie. Game must be quit from the app, or force quit, and then all is well again. Something very evil going on in Unity, creating blocks at the system level???

ETA: Seen on both the current (772) and previous build.

ETA2: I haven't seen the random hard crash problem since this one showed up, IIRC. Another symptom of the same underlying problem?