[LAU] Glitch when encoding a wav file to mp3 with lame

Hi all, I've a little problem encoding an improvvisation I recorded in ardour2 with lame. After exporting sucessfully the two tracks, I run this command: lame --preset standard mixdown.wav The generated mp3 file is mostly ok, but there are some glitches as you can ear at 1'37" (mp3 link: http://persbaglio.it/downloads/improvvisando/risveglio.mp3 ) I've done this process twice, with the same results and I'm almost sure that there isn't any glitch in the mixdown wave file (I can't be really sure just because I don't have the original here at hand). Any idea on what could be the cause? Thanks, Andrea P.S. My system is a lenovo ideapad s9e (atom n270) with debian sid, jack 1 and an audiofire2 audio card.