I would guess that you have a corrupt frame in your movie - what happens is that basically all the frames after that are also read as corrupt until the next keyframe.
If this is the case you'll have to repair all the errors first, you can find heaps of guides/tutorials at
http://nickyguides.digital-digest.com/
http://divxstation.com/articles.asp
http://www.divx-digest.com/index.html
http://forum.doom9.org/archive/index.php/
It can be really frustrating though - do you need to add the subtitles? You can always use
VSFilter, much less hassle
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