It is great that you notice. Observation is the first (and maybe only) step to dissolve your worrying. I dont think is a cause but a response of your mind perceiving a threat. I mean that is not because your throat get tight that you stutter but because you dont want to risk the content of what you are saying that you uncounsciously tight your throat. I hope the explanation did not get to tangled. Next time it happens just notice that is tight. The notice itself will relax the muscle. You cant be observing your muscle and being tense at the same time. At least you are doing on purpose. Or make it the other way around. Contract the muscle voluntary. You see, if you want to tense voluntary your muscle you have to observe and focus your attention on it. In the end is the same exercise. **Observation is the key.**