Unusual sound problem

I'm not a newbie, I've been making Flash productions for while, but there's a problem that in the past I've attributed to my old (slow) computer.

In my cartoons, the sound slowly slips out of sync. After a minute or so, you start noticing that the sound is playing a little before the picture, and it gets worse the long you watch, unless Stop it then Play it again, which gets the sound back in sync.
It affects all sounds, no matter when the keyframe that starts occurs the sound in the movie.

I've gotten into the habbit of making all my sounds as Stream so they keep the picture in sync with the sound for slower computers if the picture lags. Could this be causing the problem? (I'm using the default 12fps by the way.)

I would appreciate any suggestions.

Movies longer than 1000 frames?

I haven't counted but yes they are I think. My new one is definetly.

http://www.markhaldane.com/flash.htm Elwood & the Fly has the problem in it I believe, to a small extent.

I think I've solved the problem. I've set all my sound effects, dialogue and music to 'Event', and to keep it all in sync, I use a short silent 'null' sound looped for the length of the scene, and set to stream. It increses the file size a little but it's worth it. It also means I can create stereo effects with my sound, something I couldn't do with streamed sounds.