Speech is very context-dependent. Speech performance is often impacted by pressure or anxiety. For just about everyone, speaking in public carries much greater performance anxiety than speaking aloud at home, which often leads to more stuttering. If you're struggling with vowels, you can try using [easy onsets](http://www.youtube.com/watch?v=tfvgX7uW3vA). This is sort of like putting a "silent h" before a word, e.g. "(...h...)apples" instead of a hard " 'apples" (I say "silent h" because you don't want to say "happles"!). The idea is to release some air before you begin voicing, so that the air is already moving once you do start to speak.