Download a file then the next error message appear The connection with the server was reset

The application is coded using ASP.net and C#.
A part of the application allows the user to download files. The size of
these field can be up to 80 MB.

It works when the connection bandwidth is high. But in other cases, after a
few minutes, the next messages is displayed: "The connection with
server was reset".

Here the code that I use:
private void SendPageToBrowser(string filename)

{
// All mime type supports
StringDictionary MimeTypeList = new StringDictionary();
MimeTypeList[".doc"] = "application/ms-word";
MimeTypeList[".zip"] = "application/zip";
MimeTypeList["*"] = "application/octet-stream";

FileInfo fi = new FileInfo(Server.MapPath(filename));
// Checks also that the file exists before cleaning the response
// in case, it doesn't exist, an exception is raised
string FileLength = fi.Length.ToString();
Response.BufferOutput=true;
Response.Clear();
Response.ClearHeaders();
Response.ClearContent();
Response.ContentType = MimeTypeList[fi.Extension];
if (Response.ContentType == null)
{
Response.ContentType = MimeTypeList["*"];
}
Response.AppendHeader("Content-Disposition","attachment;filename=" +
fi.Name);
Response.AppendHeader("Content-Length", FileLength);
if (fi.Length < LOWER_LIMIT)
{
Response.WriteFile(fi.FullName );
}
else
{
Byte[] Buffer = new Byte[CHUNKSIZE];
FileStream DLFile = fi.OpenRead();
while ((Response.IsClientConnected)
&& (DLFile.Read(Buffer,0,CHUNKSIZE) != 0)
)
{

Response.BinaryWrite(Buffer);
Response.Flush();
}
DLFile.Close();
}
Response.End();
}

What's wrong in my code or which settings of IIS I have to change?

Any helps will be greatly appreciated.

Olivier.