I recently bought a 1950's Conn Naked Lady sax and it works fine except the low D and E are, for lack of a better word, "bubbly", or unstable. It's not a leak because C and below sound fine. I've had it checked out and he found nothing wrong, so I assumed it was operator error. I mentioned it to a friend who asked what kind of mouthpiece I was using (it happens with both my metal and my hard rubber ones.) He said the problem was the shape of the instrument and suggested putting the plastic cap (or some other sort of foreign object) down the bell to create back pressure. That helped slightly, but not a lot. I've never had this problem on other saxes (Selmer Mk VI and Yamaha YTS-23), so I'm not sure what to do. What can I try?
EDIT: Here's what it sounds like: http://loami-windmill.com/Music/StackExchange-Bubbly%20Tenor%20Sax.mp3