Files can get "stuck" as you've seen, for all the reasons that people disconnect form the network. I've read of people asking for an option to "pause" downloads--maybe you hit a servent that has implemented a "pause" upload.

You can view the vendor/IP information for the file--control click the column headers to see those options. Maybe you hit a 'bad' or old vendor. This won't be conclusive, but does give more info to wonder about.

There are a couple of 'oh well' tricks to babysitting a file. Enter a new more specific search to revitalize that file; also try the Resume button. Too, delete the file from the Download Pane, then go to the Library Pane and select it from the Incomplete list, and hit the Resume button to send it back to the download pane. Maybe File->Disconnect then File->Connect will get you a better set of hosts.

Lastly, don't trust the numbers too much, especially on large swarmed files. Since the file is downloaded in 100K chunks, there are situations where later chunks might be completed before earlier ones, or some chunks need to be replaced. Maybe you were getting the fast earlier chunks from other who too are waiting for the last key piece which could originate from a little old modem user!

Is this sort of what you were getting at?