Problem: FTP transfers occasionally slow to a crawl, once the number of bytes transferred exceeds 20 kilobytes (20k).

For users on a Windows platform, this problem can often be fixed by setting the MTU (Maximum Transfer Unit) in your IP stack/driver (Internet Network Access software) to 576. The default is too high in certain cases. Please contact your Internet Service Provider or the company which provides your Internet Network Access (TCP/IP) software for further information.

Also, http://www.gifart.com/modemtips.shtml has instructions for changing your MTU.