What you've described is pretty classic stuttering. Increased likelihood of disfluency in conversation, particularly when the "stakes are high." And decreased likelihood of disfluency when alone (if you have pets or small children in your life you likely will stutter less frequently with them as well). It's not likely that you never stutter when alone--you probably still do--but you're likely not as "tuned in" to your speech so you miss the little ones. But it is fascinating. The reason for this is potentially the "propositionality" of the language. Propositionality is a complex way of describing language that encompasses semantic meaning, syntactic structure, pragmatic inflection, content familiarity etc. So when you're talking to a person you want to (for a lack of a better word) impress, or talking about a content area you're not particularly familiar with, or using more complex language you're going to be more likely to stutter. The reading out loud and public speaking are interesting to me. Do you mean whenever you read out loud, or when you're alone and you read out loud? How structured is the language in your public speeches? Do you memorize them?