Fast at Shows

My horse, Mandy, is an ex-barrel horse, albeit she was a slow one, but I've spent the past two and a half years slowing her down and making her into a pleasure horse. And she has adapted wonderfully. I've even turned her into a showmanship machine. But there is just one major problem; her lope. Her trot is wonderful, like riding on a cloud. But she sometimes wants to get up and go. She 'canters' and trots slower under and English saddle than western. And during lessons, and only during the show classes and horsemanship, does she go fast. I've related horsemanship to the cones, maybe she associates them with barrels and thinks she has to run? And I've tried different western saddles, which did help some. I've also tried different bits, which also helped some. When I get to the shows, she is lunged for about 10 minutes, then ridden for about a half hour. And during the course of the show she isn't ridden hard. I have no idea what would cause her to go fast so suddenly in the show ring. I'm so confused and frustrated at the sudden change in my horse when we show, because I know she can go much slower. Any and all advice is greatly welcomed and appreciated.