I hear you... It can be quite disconcerting. I feel like I don't have as much voice control as I would like. I can sound fine in rehearsals, but then on the day I sing the pollen count will be up, or something. What works for me (as Seamus mentioned earlier) is to avoid milk and dairy on performance day, or maybe the day before, as well. I think it helps me too, to stay away from sugar and carbonated drinks before a performance. I like to stick with water and tea if I can.

I tried taking a decongestant before a show once. Disastrous. TOO dry. Find out what works for you. Good luck. Sharon