Performing issues-mess up on pitch up only on stage?

It may be nerves more than you think. I've had the same problem. I've never had a problem with pitch in my life but when I get nervous I've known myself to get slightly pitchy. This is due to tensing up. Tension in your vocal chords/throat/body will cause strain, thus not allowing your singing to flow properly. Just make sure you are relaxed before a performance. Calm yourself. Break a leg!