Do you notice your approach being slower or faster in league? I would suggest that when you throw your practice shots in league. Take the last couple of shots to throw the ball aggressively so that way it gets rid of that amped feeling and now you are calm and relaxed and ready to do what you know how to do.
Coach is right about your grip. I had that happen to me. I asked the guy in the
Pro Shop to open up the thumb hole a little because it was tight and he opened it too much, so I threw a lot of bad shots because I had to grasp the ball so that it wouldn't slip off my hand.
Again, I would suggest the brain-dead as possible approach, where you don't think about anything up on the approach. If that doesn't work for you then a song might be a good way to block out everything, especially one that works with your bowling timing. For myself I like Requiem of a Dream or its remix works.