Why does it come back sometimes ?

Why does it come back sometimes ? When talking to strangers over the phone, and to my close friends, it is fine, I could talk for ages and for however long. However, when talking to family members, strangers outside or speeches, I stutter? I don’t know why ! I articulate myself well when by myself and when talking to close people but with others I just sound stupid. It’s so annoying because they just stare at me like I’m a fool. When I was talking to my sister I couldn’t even speak properly. Idky. Embarrassing.