Adam/ARG - I can (probably) fix your yawning problem. Some of my students (martial arts) have this happen to them on occasion in the middle of workouts. Used to happen to me too. Anyway, the reason why you yawn in this case isn’t because of tiredness but because of a lack of oxygen.
Answer: Take a breath! I bet if you stop and examine your behaviour when you are nervous you will notice that you will tend to hold your breath more.