This happens to me to when I speak Japanese (my native is English) When speaking English, I find that I stutter more saying names, titles, etc, because there is more pressure to say the word exactly how it is. I can't use synonyms to help me. With another language, we focus a LOT on the words we say because we are translating each word in our head a lot of the time. Because we focus on each word, at least for me, it causes a stutter in the same way. Pressure to use the right word makes my stuttering worse, in other words.