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Like Al & Carol said, there is a tone of stuff on this subject in the archives. In short, because of the geometry of the way a fifth wheel attaches to the Tow Vehicle any motion that affects the level pull ( ie trailer verticle motion or truck rotating about either axle ) will result in the pulsing motion you are referring to herky jerk.
The first thing to do is to make sure you have enough weight on the pin. It should be about 20% of total trailer weight.
The second thing is to make sure you are not overloading the trailer. Just crawl undr the trailer and note how much free movement there is above the axle. The more the better but at least an inch or two of movement. While under there make sure you have trailer shocks and that they don't appear to be leaking. I have come to believe this to be one of the most deficient areas in trailer design. As Orve said, independent suspension is the ultimate fix here. You will more than double the ability of the trailer to handle rough roads.
Third, the truck and trailer should ride close to level. You solved that with the airbags. And, as Al said, make sure the TV shocks (rear and front) are in good order.
Fourth, the Mor/ryde pin box (as well as others) directly attack the herky-jerk problem by putting an elastic element in the fore/aft connection at the pin. These work quite well, in my opinion. However, be advised that these have a limited amount of movement and eventually bottom out. Get on a really rough road and they will become overwhelmed just like any other system.
Finally, probably the cheapest solution is to change speed (usually slow down) and find a better road. This usually mean getting off the interstates. Heavy loads on these roads tend to get excited by the breaks in the concrete. Over time the traffic tends to enhance the undulations. These undulations are naturally at the natural frequencies of the vehicle suspension of the vehicles that cause them. These suspension frequencies are about the same on most heavy vehicles. Good shocks tend to raise the natural frequencies of your suspension and help you get away from the road frequencies. Here again, however, eventually the undulations in the highway get so large as to be able to overcome what ever you do until they resurface the highway.
I started to develop a model to describe all of this in concrete terms and will get back to it someday. Just too many other interesting things going on in the moment. (time for a homebrew)
Dave
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