How to catch a COMException properly from ASP.NET?

We are using AD to authenticate user for our website, and sometimes the Web server is not be able to connect to the DC server, and we got a COMException (0x8007203a): The server is not operational.

Even when I catch this exception in a similar code block below, the exception is still not caught completely, and it is propagated to the Application_Error method in the Global.asax page. It happends only with this COMException, other .NET exceptions are caught and logged properly.

try
{
   // This line of code will start AD search and raise a COMException.
   SearchResult results = objAdSearcher.FindOne();
}
catch (COMException com)
{
  _handler.HandleException(com, ExceptionHandlingProvider.UnhandledPolicy);
  error = "The server is unable to process your login request. Please contact an        administrator.";
  return true;
}