I dont think I ever stammer when talking to my mum. It happens with my dad quite a bit I've noticed but it's not like I'm scared or arguing or anything. Last time I was taking him to the doctors and because he cant speak english, I wanted to ask him "so what should I tell them when I take you?" But i just couldnt get the words out of my fkn mouth. It can even happen if I talk to someone I require the help of for example a receptionist if I was returning something or asking for something etc.