I agree with hmfadopter, play slowly. In particular at the end of the practice session, play through the piece once just ridiculously slowly, unbearably slowly, so slowly you can't believe you can make yourself play it that slowly. Then leave the piece alone for the day. Of course you can practice the whole thing up to speed, or practice the difficult parts, just be sure to finish every session by playing it all the way through, without stopping, painfully slowly. Do that every day at the end of practice for a week, and I'll bet you'll be able to play it through smoothly at tempo.