Symantec and ISA Server 2000

I have an ISA Server 2000 (on a Windows 2000 SP4) and it pass throw a Symantec Gateway Security 320, and on the server I also have installed the Web filter from Symantec.
When I’m downloading a file it just stop and says that the download was completed when it was not. For example, if the file size is 500 KB, it just download 130 KB.

PLEASEEE if someone can help me with this.