I know this is a Race horse forum but I am going out of the box to see if any trainers have dealt with this problem and have anything to offer. I have exhausted advice from our own discipline/world. This is a draft horse that competes at high level in the lead of a six horse hitch. She is 5 years old. After a few laps at a full trot, she sometimes will start roaring, very loud. She never runs out of go or wind but she sure makes you think she is going to. Then when you line up, she instantly stops, and will also stop in mid stride if something distracts her or catches her attention. She scoped normal standing and also normal in harness with the dynamic scope. Frustrating! We have tried the usuals like Ventipulmin, wind aid, vicks, dex injections, flushing her mouth before she drives, antihistamines, a ton of different bits with every size and shape of mouth piece, different irons, different check systems. She does not seem to fight and of them, she will just start pulling so hard and then the noise will start. We have had her adjusted for pain and magna-waved lots of times. If you get right up at her when she is doing it, we could see her pulling her tongue up if front of her airway so we tried tying her tongue. That doesn't work either. She also does not salivate which is what I am wondering may be behind the whole problem. Any thoughts from your world?