I had to convert about 100 gb on my HD to about 55gb for my iPod during autosync. In doing so, I had to use all four mp3 plugins at some point to complete the task.
Therefore, you should try both (MAD .14.2b and in_mpg123.dll) of the addon plugins-- one at a time-- and see what works.
What you may find is the two plugins that come with MM (in_wmp3.dll and in_dsmp3.dll) will convert/autosync most of your files, but the two addon plugins will hopefully convert/autosync the rest.
I'm a long-time MM devotee (at least 4 years now). But I'm at my wits end trying to sync with my new 80G iPod.
In a nutshell, the files appear to be transferred to the iPod properly (they are on the iPod and they can be played from the iPod folder in MM), but most of the tracks simply won't play on the device. As some other users have reported, the iPod appears to want to play the track, fails, and then skips to the next track. Based on several different attempts to sync different lists there seems to be a success rate of <20%.
I tried uninstalling iTunes and followed rusty's documentation, but no dice.
What's most frustrating is that the same tracks do sync properly using iTunes! Can anyone help--I'm not sure I can stomach iTunes when I know how superior MM can be.