I'm new to flash, but whenever I encounter flickering in my other A/V progs
it's usually either cuz it needs to be rendered into a different format or
because there are missing frames or both.

Also, how many frames are you tweening. In photoshop, the more tweens and
frames you have, the slower the final product runs. Perhaps you are having a
similar or opposite problem. Just tryin to help. Sorry if i didn't.