well, it looks like that the OP's problem has to do with the fact that the other encoders are using the 3ivx codec in an AVI container that's causing the problem or the variable bit mp3 file. I would bet that it's the VBR mp3 that's the culprit and it would be easy to test if you are using virtualDubMod.

for the following, you'll need to have:
VirtualDubMod
a way to make a CBR MP3
the AVI codec of your choice (3ivx, xvid, dvix, etc etc)

Load your file into VDM and go to the streams menu and pull down to Stream List. It will show you the audio stream and you save as WAV, but sometimes it doesn't work at times. Check the file it outputs as if it's the size of an MP3, chances are it is (might have the WAV extension, but it's an MP3). If it doesn't work, use the demux option and then convert the MP3 file to a WAV file... then convert it back to a Constant Bit Rate (CBR) mp3.

Back in VDM and still in the Streams list you were in before, select the audio file and then hit the 'disable' button. Then hit the 'Add' button and go to the CBR mp3 you saved, and it will add it to the streams list. Hit 'Okay' then and it will return you to the main VDM window.

Next, go to the 'file' menu, pull down to 'save as' and it will bring up menu showing the save as options. At the bottom of the window, there are two pull-downs: video mode, and compression. You can change the first one to 'direct stream copy' and it will attempt to just add the new MP3 file to your existing video file (if you have the proper codec involved, that is... I think). If you don't, you can reconvert it into another video codec you do have (I'd suggest xvid, but use what you have). Change 'Video Mode' to 'fast recompress' and 'compression' to your codec of choice. Add a name to your file and hit 'save' and VDM will output your new video file to test out.

or, you can upload it and I could give it a try for ya