I only stop stuttering when I'm angry, why?

I only stop stuttering when I'm angry, why? I stutter pretty often, when it happens, it's always on random words. One day, I just can't say certain word at all, but the next day I can pronounce it with no problem. I work in customer service, so sometimes the stuttering can be a bit awkward. Weirdly, when I'm angry or upset, I stop stuttering completely. When I'm talking to myself (weird?) I won't stutter at all. Does anyone have tips how to manage stuttering in everyday situations?