switching leads

Question
I have a very interesting little horse, with an interesting problem. In July I got a free horse, a quarter horse arab, because he needed a home and I needed a companion.  He quickly became my number one horse, because I felt he had great potential.  I am interested in dressage, and so began to rehab him (he had been out of work for one year) with basic dressage training.  He did very well, the instructor commenting that he continued to surprise her.  He was up to starting a little bit of shoulder in and leg yield, before the ground froze here and we more or less stopped for the winter, apart from the occasional thaw or beach day.  He is willing, supple, goes well in a treeless saddle and a bitless bridle (he was quite girthy when I got him, but that is less)  He does have a tendancy to veer toward the gate, popping his shoulder.  He has a marvelous trot.  He is a funny looking horse, with short legs, basically a quarter horse body with an arab way of going.  When he is working, however, he can look quite stunning as he really uses himself and has a powerful way of going.  He can stretch down to the ground while trotting and does frequently while on the lunge.  He has had a few massages.  He has had slow and careful work bringing him back into condition, and has hunter paced with no problem.  He is currently transitioned to barefoot and doing great with it. His feet are short, round, with wide frogs, and he truly lands heel first.  His problem is lead switching, behind, while being ridden, and while being lunged.  He can go five or six strides at max, before he pops into the wrong lead behind.  He never gets the wrong lead in front.  He will often correct himself behind and carry on, or break into trot, correct himself and continue.  Even while galloping he switches.  We had been working on lots of transitions to strengthen him, and felt that he would improve. However, he really has not, and I was disheartened to finally get a hold of his previous owner who said that he always did that with her, and that he never really got over it.  We don't know much about his background, he was bought from a dealer at about age 12, and his previous owner had him for 5 years, she showed him low level western shows, but her trainer did some dressage with him, and felt that he had some dressage background.  I would like to know if anyone has had a similar problem, and if it was resolved, what is the best exercise I can do with him, and if it is likely to be from pain or a physical limitation.  He is fabulous to ride, otherwise and quite athletic, quite flexible, he has a great attitude and has been a lot of fun.  any help greatly appreciated.  

Answer
Hello,
I am gessing that he is older.  If he's like most quarter horses, he has a low head set, down hill and also has a long back.  All of these can be factors to switching leads behind.  I have a quarter horse very similar to yours.  What I have found to help is doing lots of half halts.  Half halts are very useful to shift balance off of the forehand.  Be sure to make sure he is active behind and is off the forehand.  As for his back, if its long it also could be achy.  You might try a supplement.  You could also have a chiropractor come and see if his alignment is ok.  When you are working in canter be sure that he is off the forehand.  When he switches leads, stop and do not accept the wrong lead.  Be subtle, don't make a big deal about it just "fix" whats wrong.  Our chiropractor showed be how to "pop" their backs, use your fingers and draw a line from the point of the hip to the top of the tail. Half way between those lines, scratch with your fingers, or even quarter till he raises his back.  This is something you can do before you ride.  It sounds like you are doing a good job on this guy and progressing quickly.  Good luck, and if there is any more questions, just ask.  Let me know how it goes!
Thanks, and happy riding!
Kathryn