Who's advising you about these experiments? I've never heard about "speaking with your mouth wide". - Anyways, being anxious might be a big factor for some, but not for others. Understanding stutter variance is so far a fickle project, because it proceeds so differently across stutterers (and within individuals as well). Figuring out our influencing factors, and addressing them, may help us for a while, but there's no solid guarantee it'll keep working. Better to keep working on managing the stutter itself, instead of trying to avoid it (which is most likely very fickle).