nipping/biting during agility classes

Hi there,
we have a two and a half year old Rescue Staffy (got him when he was one year and 4 months old) and decided to join agility classes to give him some additional exercise. He does quite well in Safety-Road/Obedience classes and overall he is a lovely dog, enjoying playing and being obedient at the same time. There are usually no problems with other dogs (just the occasional odd one), so that we keep him mostly off the lead and everybody in the neighbourhood (both with and without dogs) know and like him.
Here is the thing now: In these agility classes, he literally transforms into another dog, reaching an excitement level we never saw before, sometimes already with the first exercise (just after 5 min). In this moments he nips our clothes, pants or jacket, and doesn't let go. It happened already, that by doing that he left some very nasty bruises on the leg. This behaviour is independent of who is doing the class, my wife or me. My wife started doing agility with him, but because it didn't work, I gave it a try, but with the same result. It's frustrating to see him in this condition, since everywhere else, even with loads of other dogs and people around, he never gets to this excitement level. Our trainers are unfortunately unexperienced in terms of Staffies for Agility, and actually at the moment we decided that we better stop to not put him anymore in this situation. But we think we need to figure out the reason for this massive behavioural change that happens only in these agility classes. We used lately only food as reward, as he never gave back the toys in these classes. At home or at Obedience classes giving back toys isn't a problem.
Please let me know if you have any advice or explanation for that. We do not know what happened in the first 15 months to him, but he sometimes flinches when I just lift a hand.
Thanks in advance
Stefan

Hi I do agility with mine I don't have this issue but some other people do in my class and it is all excitement based and usually a confusion thing as well like they are not sure what is coming next or what you want them to do so they will jump up and nip. have you tried distracting with food to redirect the attention?

Agility is an extremely exciting environment for any dog to be in. Katy gets very vocal when its not her turn, so I distract her with treats. Sometimes she gets over excited & jumps up at me rather than run the equipment properly. Its just over stimulation, I take her out of the area until she calms down & focuses on me.

A friend had this problem with her collie and, as kattie says, it was all excitement with a bit of confusion thrown in for good luck. She used a mild aversive (a clicker, believe it or not), and within a couple of sessions it had stopped. Can you slow it down for him, that might help a bit.