I don’t have much experience with public speaking, but I used to have this problem on occasion when talking on the phone. I I think it ultimately boils down to being relaxed vs nervous. You don’t normally run out of breath in your everyday interactions with friends or family, right? Why should it be any different when talking from a stage? In both situations, all your doing is communicating. It might help to prepare and practice your presentation until you’re blue in the face. Do it dozens of times until you could give the presentation in your sleep. When you get that comfortable with what you’re saying, you’ll have all your breathing pauses memorized and there will be no reason to feel anxiety anyway.