Feep,
the porpoising effect that you described.... could it have been anyway related to how the plane is trimmed, thrust line of engine, or incidence angle of the wing? I hope I am not building a porpoise, or a rock, or any other non-flying entity!

If only I had waited a couple of weeks to design the tail: [link=http://www.theaerodrome.com/forum/showpost.php?p=233383&postcount=817]construction pics of monoplane elevator[/link]