What type of web server, and server side coding platform are you using (if any)? Is any of the content coming from a backend system/database to the web tier?
Are you sure the problem is not with the client code accessing the file? Most clients have timeouts and downloading large files over the internet can be a problem depending on how the server behaves. That is why file download utilities track progress and download in chunks.
It is also possible that other load on the web server or the number of users is impacting server. If you have little memory available and certain servers then it may not be able to server that size of file to many users. You should review how the server is sending the file and make sure it is chunking it up.
I would recommend that you do a HEAD request to check that the URL is accessible and that the server is responding at minimum. The next step might be to setup your download test inside or very close to the data center hosting the file to monitor further. This may reduce cost and is going to reduce interference.