you could try audacity
http://audacity.sf.net
your problem is fundamentally flawed.
mp3 does not allow you to just.. chop things in half and get away with it like you can with a wav file. mp3 is encoded.. all the little bits are related. with that said your splitting using Peak is re-encoding that mp3 already. the mp3 has to be decoded, then reencoded after being split up into your parts. just the way it is man. mp3 doesn't allow you to just edit or chop, it has to be converted to wav, then reencoded back.