I think you might need to look before the flutter for answers. Fluttering requires an air flow and tongue position that is already present in my air stream. Maybe that's not how "your" airstream is focused? There are may ways to produce sound on the sax. Or different schools. Hak mentioned himself having difficulties because he comes from a classical school. I do as well, but I'm sure a different classical school? Make sense? At U Miami in the 80s, my teacher changed my entire air stream focus my freshman year. Forced me on a C*, and had me push waaaaaaay in, in order make my throat open up to bring the pitch down. I hated it then, but can appreciate it now. When I flutter up high, I have to open up even further. The tongue goes a little farther back, which lowers the pitch. I think? Hmmmm. When I get the horn out today, I'll analyze it. It might raise the pitch, which requires relaxing the jaw even more. To be honest, it's so second nature now, I don't even think about it. Which is really the whole point I suspect. Just don't give up. If you like that style, it's a great trick.