If she does it but not while you are on her I'm going to suggest you might be the problem.  I'm not saying this is bad but you can learn from it.  Are you SURE you are sitting exactly in the middle of your horse?  Do you cue the same for each lead; sounds, reins, pressure and position?  Do you shift your weight more on one lead or the other?  Could outside distractions be influencing your riding sessions...like she wants to lope towards the pasture/barn/etc?

If all the above answers are really NO, and you have had someone watch just on the off chance you are doing something you don't realize(bad riding habits are notorious for causing this)...does the tack fit the horse properly?  Could there be a problem with something flapping or pinching?  Does the bit/bridle pinch her face and she tries to lope out of the pain by going right?

Once you have gone through all your tack...then I'd look for a physical cause.  A sore rib, back, neck muscle, leg, knee, hock, hoof, shoe, ear, etc.  Leave no hair or hoof unturned!

If the horse is between 3 and 5 you might be looking into a hock problem, more exactly a hock fusing problem.  It's not a problem when she's baring her own weight just when you add to it things get ouchy.  I had a Mighty Awesome paint gelding and he had this...good news, 99.9% of the time once the fuse is complete everything is back to perfect.  But I recommend checking everything else first not only because in my experience is that most often the cause but it saves you the cost of X-rays until your almost certain the hocks are the culprit. 

You might try having another equestrian ride her to see if they can feel the problem...

I'm coming back as an Amatuer when I start showing again.  I trained for a short time and my students did well, but the parents drove me NUTS!  I haven't trained since finding one of my recently divorced (horse)parents passed-out drunk in my barn one night.  The straw that broke the camel's back...I gave her 2 weeks to find another place or I'd help her finish her barn and move her horse home.  I finished her barn and she came to get the horse but couldn't load it.  I watched for 2 hours while they fought to load.  Finally, I walked over touched the lead and shot the horse in the trailer with only one cluck.  I felt really bad for the horse and the little girl...after that they trashed that horse.  I closed my gates and don't train any more.  I genuinely LOVE the sport and if I'm in a town where there is a show you'll find me there in the stands cheering for the little kids and sympathizing with the trainers as they throw their hands in the air when kids blow a patern.

I've moved and now am about to finish my new home in south central Oklahoma.  Once I've had the kids and all that jazz I'll be back on the scene and most likely showing under a long time friend that showed/trained with me as a youth, knew me as a trainer...and yet after all these years is still someone I trust in the horse industry, Mason Lyon.

So my advice is free and may only be worth what you paid...

If you need anything just ask.  I'm always giving pointers and answering questions, just remember it's worth what you paid.  I don't touch horses I don't own and while I miss the kids and the horses I don't miss the parents and their issues.  My day will come again...until then I'm just a lady that knows horses.

And thank you for the compliment.  I don't train any longer.  I've done just about everything but cattle classes and speed events.  I love the english events because I feel like you ride a more natural horse, don't get me wrong I love a good pleasure pony also but there aren't many born that way.  I honestly feel a well trained English horse is more versitile than a well trained pleasure pony...think Rugged Lark, Gifted Circle, Artful Investment and Acadamosby Award.  Trainers are a dime a dozen but good ones are hard to find.  I'd stick with someone that is competitive and trustworthy.  If you want to stay current keep watching who's winning consistanly, anyone can have one good ride.  Reading the rule book won't tell you what judges will pick just what they should.  You must learn how horses are judged and start thinking that way.  Your local college may have a horse judging team.  See if you can watch some of their judging tapes, place the class and listen to the expert reasons given after.  Once you know what makes one horse better than another you'll know what exactly you need to do when you are in front of the judges because now you've learned how they think.  (You can do everything a trainer tells you and still not "get it" if you don't know what the judge is looking for and like I said the rulebook isn't going to teach you, chances are neither is your trainer.  I had my kids placing and explaining classes to me while they weren't riding so that they could visualize what we were trying to accomplish in lessons.  I recommend this to anyone that competes...if professional athletes do it so should you!)

I'll be a good parent!! Promise!!  

I still wish you were closer.  My daughter and filly are using a lady who judges Apps, paints and QH's.   Hopefully they will keep their heads come show time and not panic.

At lessons I just sit down and read a book, or since the trainer is having me bring my old mare for my youngest, I supervise her in the other arena.  The lady I got 2 of my Apps from used to train/give lessons and she quit because of parents too. 

Boy, I sure wish you were still in the area and giving lessons.  It's tough finding someone.  In the past two years, I've been trying to find someone for my little niece.  I had a hard time getting anyone just to return calls.  It was like they didn't really need the business.  Then when we did find one, their schedules were so sporatic I couldn't get the kid there often enough to keep her interested (fitting it between all her other activities).  Now she's not even interested.  

Now that I've got my new guy to get back into riding, I'm trying to find someone for me (well, us...), but finding a good balance of teaching capability, availability, location and affordability is tough.  I actually tried one instructor that told me that he didn't make his students wear helmets unless they were jumping....