This is happening cus you are stuttering and getting out of breath. My advice would be to stop and take a deep breath when this happens and try to recuperate. Years ago I presented at a poster symposium which was basically 2 hours of constant talking. Near the end I was lightheaded and close to fainting because I was stuttering so much and that caused me to not be breathing as much as I should have. That was where I learned that if I ever feel that way or if that my chest is tight, I need to slow down and take some breaths.