I believe your downloads will disappear from the download window if you do not properly close Limewire, i.e. if you turn off your computer without closing Limewire completely. To finish your incomplete downloads, open the Library tab and go to the incomplete folder. Right-click on your partial files to restart them (I think you go advanced...search for more sources...).

Do the songs you're downloading play better or worse depending on the bitrate? The higher the bitrate, the better they should sound, but not always.

Good Luck

Dano