This sounds like a classic case of a horse's teeth not being balanced. If the teeth are not balanced, then the whole horse will not be balanced. Does your horse tend to not pick up one of the leads as well as the other?
I would HIGHLY suggest that you have one of Spencer LaFlure's certified dentists out to look at your horse. Spencer has a different approach to floating teeth, and let me tell you, no one else will ever float my horse's teeth. The changes in him, along with all of our friend's horses, have been phenominal, and immediate. I have had 2 vets tell me that they spend abolut 1 day learning how to "float" teeth. Sure, they can identify pathology, but they don't know how to fix it. And a lot of equine dentists take off too much tooth (especially if you have your horse floated twice a year) and they don't know how to properly balance the teeth. Spencer's website is www.advancedwholehorse.com
If you want more info. on him, feel free to ask.