a/v sync problem when muxing/merging with avimerge/mencoder

hi there,

my problem is not directly related to dvd:rip, but as you are familiar
with the linux video/audio tools i hope you maybe can help me.

i'm searching for a possibilty to merge/mux an avi with 2 mp3s. at the
beginning i have an avi with one audio track and a separate mp3.

what i want is, that after the process i have one avi with two audio
tracks, where track1 is the original separate track and track2 is the
track which was original in the avi.

at first i tried avimerge alone, but got avilib errors telling me that
the avi is not an avi file.

then i tried the following:
mencoder avi_file -audiofile separate_mp3 -oac copy -ovc copy -o temp_avi
avimerge -b 1 -o two_audio_file_avi -i temp_avi -p avifile

this completly runs, but gives me an avi in which both audio tracks geht
more asynchron as more of the avi is played. since both tracks divide
the same way, i think there must be something wrong with the mux/merge.

till know i used virtualdubmod with wine and it worked quite ok, but i'm
searching for a command line solution, to accelerate the process.

both avis are vbr, if this changes anything.
is this maybe a problem with mencoder/avimerge and i have to use ffmpeg
or vlc?

i'm quite clueless, since it's working fine with virtualdubmod and it
must be something with the settings.

hope you have an idea,

regards