This seems like a rider induced problem. I would like to know what kind of
saddle you are using. Are you using a breast collar? Type of bit, etc.
When going uphill most of the weight is going to transfer to the back legs.
The rider can still interfere in the front if they are shifting their weight
to far foward. The saddle will also tend to move back, this can interfere
with the elongation of the Lattisimus Dorsi, which attaches to the upper arm.
In the trot the hind end undulates, rising and falling side to side, in the
canter or gallop the hind ends sort of folds under the horse with the hinging
in the lumbar area. The horse could be trying to gallop to change the body
movement to escape pain or speeding up because it is off balance.
If you could provide alittle more info. it would help.
The initial problem could have been due to the sore frogs, stone bruises, now
you may be dealing with the compensation for that.
By the way, have you or the primary rider had any recent injuries?
jim pascucci
Certified Rolfer