sometimes, if i am nervous - i forget to breath completely. i've almost passed out from this phenomenon. deep concentration has to be moderated with interspersed pragmatism.
my dad used to tell me when he'd walk by me practicing, 'keep your mouth shut, or you'll catch flies.' (he never said anything about the music). at least i have a tape of murray perahia with the same problem. i learned to shut my mouth and breath through my nose - but then - now i have this problem of not breathing in difficult passages.
it's only when i'm relaxed that i breath properly.