Where are you downloading it to and from? What speed is your connection, and could it be "breaking up" somehow? Have you ever had problems with any other downloads? How long is it taking you to download that one file? If it's longer than 3 seconds maybe my keep-alive timeout is causing the trouble, but I doubt it...
Try downloading it directly to your server somehow? If you have shell access, use wget.
Lastly, I can email it to you. Send me an address at trisweb at this domain (zenphoto.org).