AS3 Alpha Tween "shimmer"

I’ve got a working alpha tween in a 31 fps movie clip that fades from 0 to 1.0 over 2 seconds, black background. The images that fade in seem to “shimmer” as if the alpha blend were computed with a low-quality (high speed?) algorithm. Is this normal, and is there a way to smooth out the tween?

Thanks,

Hunter