I have this same problem with Mitchell, and a friend suggested to me, to lunge him on the flat, and slowly add poles around his circle so he had to lift his legs to get over them, do patterns with them also. say have two poles on the flat, and the next pole raised slightly, but not high enough to need to jump, then the next few poles on the ground again... If that makes sense.
Mitch also has a longer back like you say your horse does :)
^This helped Mitchell a lot and his gaits are a lot better now and he can do smaller and larger circles. However I need to start him on that again once he comes back in from his hoof abcess.
Good luck :)
R.I.P ~ Bubbles - 25yo tb mare - 13.04.2011 ~ 8:30am ~ passed away naturally and peacefully in my arms