Because they aren't available in the US through any legitimate means, I downloaded copies of seasons two and three of The Sarah Jane Adventures from a less-than-legitimate source recently. Since these were AVI files, I used a utility to convert them for import into iTunes. Just now I finished watching the first episode, or part one of the first episode, and noticed that the audio kept getting further and further out of sync with the video. I then started watching the second episode and noticed the same thing.
When I mentioned this to Tara, she remembered that the same thing happened on one or both of the similar Dr. Who downloads I'd made (one before the 9th Doctor episodes premiered here in the US, and one when we managed to miss the last episode both of a season and of a multi-part episode due to a move or similar occurrence).
It was then I realized why this happens - the difference between the PAL and NTSC frame rates. I'm betting that during the sequence of pulling these off of British TV in PAL format and converting them to either DVD (for the Dr Who episodes) or iTunes for play on a marginally NTSC environment (NTSC from the DVD, on a US based computer from iTunes) the video is getting converted frame per frame, but the audio remains at the same rate. So over time the audio and the video get further and further out of sync.
Now, if the problem isn't actually present in the Divx encoded .avi files, then there may be a better way to get it converted to play on my NTSC set systems. But if the problem exists in my source material then I'm probably stuck with it. (While writing this up I did find a frame-rate setting in the video convert I used and I'm seeing if it helps - but I'm not remembering which frame rate is which, so I may have just made the problem worse)