Although I am not familiar with gaited horses (none in my country :) ), sounds to me that the horse is lacking balance and confidence in this transition, and is falling on his forehand, causing frantic, speedy gaiting/possibly trotting sometimes? The answer to this mostly is - transitions, transitions, transitions. Canter for 5 strides, transition, gait/trot for 5, again, canter, count to five, transition. Rinse, repeat, do this after a good warmup both in the arena and in the trails.
You will also benefit greatly from pole work in the arena, hill work in the trails (or just walking over fallen logs/deep moss/any uneven surface in an active pace) and I deeply recommend straightness training exercises from the ground to build up that power in the hind end and a natural collection.
I have come a long way, to surrender my shadow to the shadow of my horse.