Finishing the stroke whilst breathing is becoming the key front crawl technique point at the moment. Last week it was the arm opposite the breath that was lazy, now focussing on that, the breath arm is short. Every so often during the evening I managed a few strokes where both arms were fine and achieved a breath, but they were the minority. Also how frequently to breathe? My strokes are neater the few breaths I take, but of course that isn't sustainable!
I was thinking that I was merely running out of oxygen but this article suggests that the muscles themselves may be fatiguing with lower breathing frequency; that's not quite what you might expect. Of course, I'm swimming slowly and not in 200m bursts, even though it feels quite intense to me.