How much of your general practice is on a mound? They really are 2 different things. Warm up and getting your arm ready is fine but having to locate the ball from a mound over a specific distance needs that very kind of practice. Having to do all of that can change everything, especially when the effort level goes up. Is there a practice mound that you can get ready from? If not, you should really do what Roger said and trust all of the practice that you have done from a mound at game intensity. If your body hasn't seen that scenario much and you all of a sudden are thrust into it in a game, it can not be expected that you'll just immediately adjust and be able to do it.
So, you need a lot of general practice in similar to game situations and then trust your practice. If your practice is not like the game situation, you're not maximizing your ability to perform when you do get there. Your body hasn't seen it and done it enough.