I compute the HD cache every frame to avoid problems, I think HDCache is not yet though for precomputation for animation and moving objects, anyway the HD Cache precomputation does not take too much time usually (at least in my scenes).
To avoid flickering I do what was posted in another thread, I increase interpolation accuracy, I'm not pretty sure on how to evaluate that settings but I'm pretty sure that if I put from 60 to 80 in position I won't get flicker, try it with some test, it's a bit slower that using 20 as default, but yet you get a superb speed improvement over plain PT and avoid flicker.
Hope this helps!
Cheers.