First of all, check the Info on the downloaded song (File -> Get Info) after you select the track, or <cmd-I>.
Make sure in the Options tab there's nothing odd about the Start Time and End Time. While you're there, go to the Summary tab and note where the song file is stored on your hard drive.
You might try moving the file from your iTunes folder on your hard drive to a safe location, Then remove the file from iTunes (iTunes should show the little grey exclamation next to the song since the file's not where it expects any longer). Then reimport the file <cmd-O> from wherever you moved it. This is just in case it is the Library that's messed up and not the download.
If that doesn't change things, then the downloaded file is the problem and you can contact Apple. They've been good for me when I've had download faults.