Avidemux wont save the whole video

[UltraNic]

Hi Guys.
I recently got Avidemux because of my fraps recordings are about 200gb large and I wanted the files simply a little smaller ...
Works with Avidemux actually really great! No quality penalty or anything. Now I have only one or two problems:

If I choose the MP4 Muxer as the output format, then after a time an error message occurs: "Too short. The video has been saved but also to be incomplete"
In a forum I have read that MP4v2 could fix the problem. So I chose it and works fine, but my video (which is about an hour long) is already stored after 12 minutes. How can I change this?

[eumagga0x2a]

Please decide whether to continue in http://avidemux.org/smif/index.php/topic,17607.0.html or here. In any case please address the requests.