During lunch, I checked LJ from the office, and found that the bonk-to-the-head stage of the animation had gone all jerky. Suspecting that it might be a browser issue (Firefox 3 at home, IE 6 at the office), I did a few Google searches and discovered that IE's rendering of animated GIFs automatically increases any frame of less than 6/100s of a second -- not to 6/100, but to 10/100s. That's just enough to insure that any frame that was assigned a fast turnover because it's critical for smoothing the animation will create a moment of visible jerkiness.
I'm trying, and failing, to think of a reason for this other than somebody actually going out of his way to be extra inept.
Retweaking the animation is trivial, but will probably have to wait till tomorrow.