I went to an amazing therapist who trained in NLP (Neuro Linguistic Programming) and spent two years unpicking my stutter and the negative processes that happen in environments/conversations where I stutter. Not only do I now live a 90% stutter free life, I've understood myself a lot more and have become a healthier person emotionally! So grateful for her and having the courage to go to therapy. For example; up until the age of 24 (when I saw this therapist), my stutter would be crazy around my parents. Turns out, because I've never really been fully accepted by my parents and every-time I talked I was subconsciously trying to prove myself (the inner child screaming, I'm somebody worth accepting!) and I could hardly get my words out. Now that I've learnt to respect and love myself, and most importantly, recognise that I don't need to be accepted or approved by ANYONE, even my parents, my stutter has reduced massively. This taught me how negative or positive thought patterns can influence speech. In your case, I had similar experiences when I was angry and expressing something I deeply care about, then my speech was perfect. But I when switched back to normal conversations, terrible. The way I understand that now is: during those moments you are connecting with a part of yourself that is true and a part of you that you respect. Otherwise you wouldn't be passionate in the first place or defending yourself. It's very self affirming when you talk about something you deeply care about or when are angry with cause, and as you let that powerful inner side of you out, your anxieties and negative thought patterns diminish for a short time. Thus less stuttering.