将Gridview导出为Excel文件，数据量稍微一大就出错，要怎么办？

代码如下（VB语言）
        Response.Clear()
        Response.AddHeader("content-disposition", "attachment;filename=" & Server.UrlEncode("详单") & ".xls")
        Response.Charset = ""
        Response.Cache.SetCacheability(HttpCacheability.NoCache)
        Response.ContentType = "application/vnd.xls"        Dim stringWrite As StringWriter = New System.IO.StringWriter()
        Dim htmlWrite As HtmlTextWriter = New HtmlTextWriter(stringWrite)
        
        Dim P1 As Page = New Page()
        Dim F1 As HtmlForm = New HtmlForm()
        GridView1.EnableViewState = False
        P1.EnableEventValidation = False
        P1.DesignerInitialize()
        P1.Controls.Add(F1)
        F1.Controls.Add(GridView1)
        
        P1.RenderControl(htmlWrite)
        Response.Write(stringWrite.ToString())
        Response.End()-----经常出下面的错误:
Server Application Unavailable 
The web application you are attempting to access on this web server is currently unavailable.  Please hit the "Refresh" button in your web browser to retry your request. 请问要怎么解决?