How do I get my sheltie to improve his contacts in agility?

I first trained him with the two-on two-off method where he would drive to the end of the contact obstacle and wait with two feet on and two feet off until released. When he was competing reliably with this method, and in the interests of faster course times, I started to decrease the amount of time before releasing him until it was almost a running contact with just a fraction of hesitation before releasing. This was working extremely well, he always got the contacts and I had 100% trust in his ability. Then all of a sudden he went from 100% contact dog to zero contacts all within one competition. He is now leaping off the contact obstacles and missing the contact zone on both the a-frame and the dog walk. I obviously need to retrain or reinforce but at training he is still perfect, just in the thrill of a competition and a new found confidence, he is leaping over them.