Maybe you're seeing some corrupted files. Does this happen with every large file that you try to download?

When the download % drops back, it can mean there's some corruption within the file...the thread below explains more.
http://www.gnutellaforums.com/155541-post17.html

Check files with Bitzi before download. People use Bitzi to rate files, so checking before download can give you some idea of the content & quality of the file.
How to use Bitzi

And this thread might help as well
How to find music(tips for movies, too)