Could you describe your Mainstage concert a bit? Which Plug-ins are you using? Do you have the same problems if using Garageband or Logic?
From the desscription above I'd look after occasional CPU overloads, which can happen while the instruments does its voice allocations, etc. This can be tamed with an increased buffer size. Check, if the I/O safety buffer is enabled. Try playing with that setting. You'll find these things in teh preferences.