dressage disobedience

Question
My 13 year old daughter is riding a 5 year old ShireX.  He is
usually a kitten to handle.  We have owned him for about 6
months.  My daughter has shown him in USPC eventing with
success.  
Over the summer, he decided to balk at the gates of the outdoor
arena.  My daughter corrects him by insisting that he move
forward.  She did add spurs.  But this problem is getting worse
instead of better.  He has the nickname of "the Lippizanner" at
the stable now.....  Now, whenever he feels like it he will attempt
to halt.  She insists that he continue forward.  She keeps as little
contact with the reins as possible, because he would like to rear
up, and forces him forward with her legs.  Sometimes his
responce is to leap very high into the air... and then continue
forward.  He also regularly will execute a pretty good trot or
canter pirorette instead of just continuing with forward
movement.  We would prefer that he just trot or canter on the
rail.
We have taken him completly off grain.  He has an everyday
turnout.  She rides with a Kimberwicke and a running
martingale.... Although she did show him in a german snaffle for
dressage without incident.
He did have an adult beginner rider (that was having trouble
posting) for his first year of riding that he had pushed around.
What is surprising us is that this problem is getting worse
instead of better.  We don't know what else to try with this
horse.

Answer
Dear Ricki:
Well my first suggestion may offend your daughter but I hope she can see the logic in my suggestion. Although she sounds like a VERY experienced rider for 13 and her objective in keeping this animal going forward is a good instinct- she is only thirteen. This horse sounds big (Shire cross) and any horse that's offering to go vertical is an accident waiting to happen. Your daughter is irreplaceable - the horse isn't.
I always begin with a thorough vet check to eliminate any sort of physical issue (teeth, back stiffness etc.) Barring this-
Personnally, I would lose the kimberwick but keep the running martingale- I'd go to a fishback loose ring if you can find one. He needs to accept contact- not lose it -though I understand your daughter's instinct to "let him go" when he gets light in the front end. Something with a little bit of bite but no curb chain. I would also put a qualified and proven trainer on this horse's back IN THE ENVIRONMENT where the problem is occurring. This animal only has to go over on top of your daughter ONE time to potentially result in serious life altering injury or worse.
Play this one safe.
Best wishes,
Shelley Evans