Agreed. n-Track uses Blade which is one of the worst encoders around...or at least it was last time I used it.
You might also look into SCMPX which does a much better job than blade, especially down at 128kbps.
As for your actual problem, if it's still a concern, you might take it directly to Flavio if nobody here has had this problem. (I don't think many n-Track users use the built in MP3 encoder)
Slackmaster 2000