I've imported a number of Gif's to a .fla file, but some of them always appear messed up in a specific way: each frame usually has the content that they are supposed to have, but some frames also have content they are not supposed to have, namely pixels from another frame. In some cases this is only a mild problem but in some cases it looks very stupid.

What's the reason for this? And moreover, is there a way to avoid this?