Therein lies the problem. Before you've even had a chance to go out and enjoy the social event your mind has created a movie or an image of you blocking or you trying to control the blocking etc. Whatever your thoughts are before the social event they're centered around blocking. You're getting what's known as "anticipatory anxiety". That's what happens when you create a mental image of you blocking be it a ast or present or future conversation. This ignites your general arousal syndrome and initiates a flight or fight response. This mental image is so powerful that it creates an intense state of anxiety and once this state has you it really grabs hold of you doesn't it? If you think about an upcoming conversation one where you would block what happens? Your mind sees the movie of you blocking or struggling to speak and thus your body acts accordingly. This process is psychosomatic because even though you may not be having that conversation at this present moment in time (let's say you're alone) you still feel the anxiety etc as if you were having it am I right?