Desynchronization of audio and video when rendering to ogv (theora+vorbis)


because I needeed to change the title sequence and adjust the levels of two sets of 10 videos, I imported the VOB files into Kdenlive and reworked them. Since the videos were meant to be shown on the web, I created a custom profile to export to ogv (theora/vorbis) that more or less looks like this:

vcodec=libtheora g=15 qscale=6 acodec=libvorbis aq=3

The first serie is OK, but some of the videos of the second serie have audio and video slightly desynchronized (a few milliseconds, but enough to perceive it). I can't explain why it is doing this. Do you have any ideas?