Hello Rancher, we all(almost all)have faced this problem one time or the other, when we wish to convey something very important to a very important person, be it your boss, your loved one or your adversary, we find ourselves craving for words, our ideas fall short of language. Not that we can't speak, but tension and restlessness blocks our mind.
I think it is more like an emotional issue. Emotions can help you express them a lot and actually they can block you to express them too. So i think this is another dilemma of the life. You cant be optimum, you can just try it.
count to 5 loudly and then say it 1 mississippi, 2 mississippi, 3 mississippi,4 mississippi,5 mississippi... I'm sure by then you'd have forgotten all your tension and restlessness, AND the subject on hand!