Re: neck problem??
Definitely check to see that the neck octave key is sealing completely.
The most common issue I see that causes this is when the neck octave key doesn't close all the way. You can check that on your silver neck pretty easily. Look at the contact point between the floating octave lever (the key that sticks up above the body of the sax when the neck is off, and lifts the neck octave key when you're playing) and the neck octave key. There should be a tiny gap (maybe 1mm or so) between the two. If there's not, then the floating octave lever may be holding the neck octave key up just enough to create a small leak.
Chazalon is right though, it could be one of many little things. Let us know what you find out after trying our suggestions - I'm sure we can diagnose this for you.