The thread does nothing after 2 minutes

I need to grab thumbnails of apsx forms which I can do with the below code. But the problem is that I have about 12000 forms to capture and my code runs for about 2 minutes and grabs as 1500 thumbnails and stops executing without giving any exceptions or errors. Please, help.

    private Bitmap GenerateThumbnail(string Url, int BrowserWidth, int BrowserHeight,    int ThumbnailWidth, int ThumbnailHeight)
    {
        this.Url = Url;
        this.BrowserWidth = BrowserWidth;
        this.BrowserHeight = BrowserHeight;
        this.Height = ThumbnailHeight;
        this.Width = ThumbnailWidth;

        Thread thread = new Thread(new ThreadStart(GenerateThumbnailInteral));
        thread.SetApartmentState(ApartmentState.STA);
        thread.Start();
        thread.Join();
        return ThumbnailImage;
    }
    private void GenerateThumbnailInteral()
    {
        System.Windows.Forms.WebBrowser webBrowser = new System.Windows.Forms.WebBrowser();
        webBrowser.ScrollBarsEnabled = false;
        webBrowser.Navigate(this.Url);
        webBrowser.DocumentCompleted += new System.Windows.Forms.WebBrowserDocumentCompletedEventHandler(WebBrowser_DocumentCompleted);
        while (webBrowser.ReadyState != System.Windows.Forms.WebBrowserReadyState.Complete)
        {
            System.Windows.Forms.Application.DoEvents();
        }
        webBrowser.Dispose();
    }
    private void WebBrowser_DocumentCompleted(object sender, System.Windows.Forms.WebBrowserDocumentCompletedEventArgs e)
    {
        System.Windows.Forms.WebBrowser webBrowser = (System.Windows.Forms.WebBrowser)sender;
        webBrowser.ClientSize = new Size(this.BrowserWidth, this.BrowserHeight);
        webBrowser.ScrollBarsEnabled = false;
        this.ThumbnailImage = new Bitmap(webBrowser.Bounds.Width, webBrowser.Bounds.Height);
        webBrowser.BringToFront();
        webBrowser.DrawToBitmap(ThumbnailImage, webBrowser.Bounds);
        this.ThumbnailImage = (Bitmap)ThumbnailImage.GetThumbnailImage(Width, Height, null, IntPtr.Zero);
        this.ThumbnailImageSmall = (Bitmap)ThumbnailImage.GetThumbnailImage(200, 200, null, IntPtr.Zero);
    }       
    private void SaveImageThumbnail(List<FormClass> objFormClass)
    {
        try
        {
            string url = string.Empty;
            string folderName = string.Empty;
            foreach (FormClass form in objFormClass)//12000 forms
            {
                InternetSetCookie(url, authCookieName, authCookieValue);
                InternetSetCookie(url, "RP_si", rsiCookieValue);                    
                filepath = "D:\\Previews\\";
                folderName = form.FormId.ToString();
                url = string.Format("http://localhost/CRM/Sockets/{0}", form.FileName);
                if (!Directory.Exists(filepath + folderName))
                {
                    Directory.CreateDirectory(filepath + folderName);
                }
                Bitmap image = GenerateThumbnail(url, 1024, 700, 600, 500);
                image.Save(string.Format("{0}{1}/{2}-medium.jpg", filepath, folderName, form.FormId), System.Drawing.Imaging.ImageFormat.Jpeg);                    
            }
        }
         catch (Exception exp)
        {
            Logger.Error("Unhandled Exception", exp);
        }
    }
    public bool SaveThumbnails(string filePath, string authCookieName, string authCookieValue, string rpsiCookieValue)
    {
        this.authCookieName = authCookieName;
        this.authCookieValue = authCookieValue;
        this.rsiCookieValue = rpsiCookieValue;
        this.filepath = filePath;            
        try
        {
            List<FormClass> globalForms = formRepo.GetForms().ToList();//Returns 12000 forms from database
            SaveImageThumbnail(globalForms.ToList());
            return true;
        }
        catch (Exception ex)
        {
            Logger.Error(ex.Message, ex);                
        }
    }

      

        
        
        
      

    

From the controller, I call this method as follows.

    public void SaveThumbnailImage(string Guid)
    {
        try
        {
            var formsService = SvcUnityContainer.DefaultContainer.WithStandardMappings().Resolve<IFormsService>();
            new Thread(() => formsService.SaveThumbnails("D:\\Previews\\", FormsAuthentication.FormsCookieName, HttpContext.Request.Cookies[FormsAuthentication.FormsCookieName].Value, HttpContext.Request.Cookies["RP_si"].Value)).Start();
        }
        catch (Exception exp)
        {
            Logger.Error(exp.Message, exp);
        }
    }