I think he's just started getting to wound up for shows. He knows it means he runs lots of barrels, and he is gate sour because heck, he knows as soon as he gets in there he has to run and turn around three barrels, which can get pretty boring sometimes.
I would take him to a show (I'm guessing he's not like this at home?) and don't even enter a barrel class. Enter a low key, gymkanna and do pole bending, a western pleasure class, heck, maybe even some jumping if they have it. But try to enter a lot of slower events. And whatever you do, DO NOT spur him into a gallop as soon as you get through the gate.
You will probably have to go to a lot of shows and stuff, but just do a bunch of stuff besides barrel racing. Eventually you can ad ONE barrel class into it, then maybe two if he doesn't go crazy, then three, but always keep one class at the show that is something compeletly different from barrels.
I've never strictly barrel raced before, but I will PROMISE you that if you don't just do barrels everytime you ride, and try different things, your barrel horse will be even better at barrels, and you will get better times. Posted via Mobile Device