Particles doing strange, random synchronized rotation

I’ve set up a scene emitting a bunch of cubes that I wanted to retain their basic orientation of square to the world axes, but the emitter keeps randomly assigning a rotation to them so it seems they pulse as they fall, and then not. Sometimes they will emit tumbling, and then, if I change a setting in the particle system, they’ll emit OK for one time. If I jump back to the beginning and start again, they emit tumbling until I change a setting in the particle system. It doesn’t seem to matter what setting, either.

OddRotationEmitter.blend (1.57 MB) Needless to say, there’s no rotation applied anywhere in the scene except that the emitting plane was rotated 180 degrees (to emit down) and then the rotation was applied. Working in Blender 2.7a on a Mac. Scene attached.

Change your cache step, for the particle system, from 10 to 1.