Inconsistent or no tone on bari

I've played alto sax for about ten years and recently acquired a Conn 'Naked Lady' sax in good condition. I had a respected technician service it and he says it's in great shape. My instructor can play this sax without much trouble. I simply can't get consistent results. My instructor says I have to mentally pitch the tone to play it, and I've been working on that and it's helped a bit. Ive also played long tones until they come out my ears and that's also helped a bit. I can easily reach low b-flat and all the notes up to about low G but when the sax 'pipe' gets shorter, the tone is either lovely, horrible or completely absent. Then I'll try again and suddenly everything works well and I wonder what the problem was. I have a solo in a concert coming up in a week and am probably going to have to bail on it. I've tried different mouthpieces, different reeds and whatever different embouchures will work. Nothing really works.

Are there any old bari players who have had these problems? Is the cure just months of long tones and practice? Or should I just shoot myself and get it over with?