在服务端下载文件提示与服务器的连接被重置。。。

我是新手，我写了一段代码是从服务端下载文件的，但这段代码在VS2005调试时是可以下载大于2G的文件，但挂到IIS6上就只能下载300M以下的文件，大于300M就提示“于服务器的连接被重置”，请各位大侠帮帮忙，万分感激！！！
这是我写的代码：
Dim filename As String
        filename = ()
        Dim destFileName As String
        destFileName = Server.MapPath("./") + filename
        destFileName = Server.UrlDecode(destFileName)
        If (File.Exists(destFileName)) Then
            Dim fi As
            fi = New FileInfo(destFileName)
            Response.Clear()
            Response.ClearHeaders()
            Response.Buffer = True
            Response.Charset = "GB2312"
            ' //添加头信息，为 "文件下载/另存为 "对话框指定默认文件名  
            Response.AppendHeader("Content-Disposition", "attachment;filename=" + HttpUtility.UrlEncode(Path.GetFileName(destFileName), System.Text.Encoding.UTF8))
            Response.AppendHeader("Content-Length", fi.Length.ToString())
            Response.ContentType = "text/plain"
            Response.Filter.Close()
            Response.WriteFile(destFileName)
            Response.Flush()
            Response.End()
        Else
            Response.Write("<script language = javascript>alert('下载出错')</script>")
        End If