How To Get a More Solid Sound Out Of An Alto Sax?

im a pretty good player but every time i sustain a note around the Bb or A area it gets somewhat of a wobble. it sounds like 2 instruments playing that aren't exactly in tune. the wobble is little but it is very annoying! i have tried firming up, putting more air in the horn, even getting a new mouthpiece! nothing seems to work! pz help!