I am trying to render some videos in H.264 format using ATSC 720p 30Hz profile. The file seems to render in a timely manner, but the playback is very jerky. If I render in straight mpeg4 format, I have no issues. The source material is originally H.264, and I am importing the clip unaltered from its original state. The source material has flawless playback, so I know my problem occurs during the render. My ffmpeg version is 0.5-svn17737+3:0.svn20090303-1ubuntu6, kdenlive is version 0.7.3. Let me know if there is other information that you need.