In fact i've discovered this problem occur only when there's action scenes with fire. I've made a backup of monsters inc and there's no problem anywhere. DOes someone know what causes this?

I've tried cce and there's no problem. Seems i've got a problem with mencoder.

How many frames are in you Gop the "keyint" command in mencoder? what kind of film is it ntsc or pal?

keyint is 15 and i'm in pal.

Can't help you then I was encoding with keyint=24 and had the same problem changed it to 18 and the movie turned out great