Same experience, during my bachelors, initially clearly able to speak "present" on my roll call during attendance, and one day suddenly not able to pronounce present, or stuttered. Shifted to "yes" word, again after some time suddenly not able to pronounce yes on my specific roll call turn. Same things in Viva examination, have to tell my roll number before starting, after some time suddenly, not able to produce that "specific roll number" of mine ,which is first thing I have to tell to him so that they mark my attendance. Same thing in public transport, where conductor ask you about your destination/stop, and you blocked/stuttered, but can speak that "destination name" between conversation. And other interesting things in that is, if I have to use these specific words in between of speech/conversation, most probably would not stuttered in these. So somehow it is anticipation thing.