Video (or picture) has limited resolution and when very thin object is travelling through screen, it may occupy different number of pixel lines. In one frame it might be displayed as two full lines and in the next one it might be smeared over three lines. The thinner the object is, the more pronounced shimmering will be. Better filtering algorithm may help to reduce the effect, but ultimately, in extreme cases like yours, there's only so much it can do. Rendering at bigger resolution, would be the best option here, because it would affect the root of the problem. If you were using time limit for your animation, then i'd suggest to leave time limit the same and double the resolution - rendered image will be much noisier, but when you'll downsample to half rez, it should contain roughly the same noise level as original frames, but AA will be much better. You can also use blurrier image filtering with bigger radius, because downsampling will make your picture much sharper and generally you don't want too much sharpness in animation.