You get proposing because the weight is on the bumper. When you hit failing pavement, most often concrete at the expansion joints, the trailer starts bouncing. The faster you go, the more bounce you will get. It's physics. You can do some things to help control it, but you will never completely eliminate it. Very good shocks, a stiff enough spring pack, more than to just barely beat payload numbers, and properly inflated tires. Properly inflated does not necessarily mean aired to the max. It means aired to match the load. Speed is the easiest, fastest way to somewhat control it. Slow is not always safe either but sometimes the only alternative.