Thank you for your question I hope I can be of some help to you.
Any horse whether PP or any other breed have limitations to their gaits, length of stride, transitions, elasticity of gait or simply their ability to remain balanced in their movement. When you say he gets “bumpy” this does not immediately point me toward the pace. The pace may not become bumpy but would definitely become side to side. If it were the pace you would be able to watch his tail swing like a pendulum from side to side. Your statement of bumpy at approx. 8 mph tends to make me think towards your horses center of balance being lost. As the horse moves up in speed to the sobreandando the center of balance begins to change. To keep our horses smooth at higher rates of speed we then need to seriously consider helping their headset and their center of balance as the rider. To keep them smoother as we ask them to increase their speed we also must ask them to bring their headsets up and our center of gravity needs to come down to help promote any horse to continue driving from the rear. To put this in simple terms I believe that as you are hitting the higher speeds your horses balance is being lost and he is heading into what we would call the Paso trote. Bottom line is he is breaking toward the diagonal instead of lateral. Remember no matter what anybody thinks the pace is the mother of gait in the PP horse. Hope this helps you and if you need anything clarified feel free to contact me and by the way Rio doesn’t happen to be a feisty almost black gelding does he?