Smart search index Error 429

I'm using partly customized search index. I'v created a handler for event GetContent_Execute and there I call my method that adds custom fields to Index. For getting content I'm using SearchCrawler().DownloadHtmlContent() method. Now, after calling this method I get exception from IIS @The remote server returned an error: (429) Too Many Requests.@ What could I do to avoid this error?