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Hi and Welcome to MOC!
From your description, it sounds like you are talking about the rubberbanding effect (as it is often called). This is a fairly common occurance on rough roads. It's also very noticeable on concrete roads with expansion joints. With a normal 5th wheel hitch in my Dodge 3500, it feels like the trailer alternates between pulling and pushing the truck. The 3575 Montana we have has less of this motion compared to our previous 5er, a 30 foot seabreeze. so, I believe the strength of this motion is based on the distance the trailer wheels are from the back of the truck.
Rubberbanding occurs because the tires of the tow vehicle and the tires of the trailer hit the rough patches, joints, etc at different times. Some of the ways that people report remedying this is by using replacement pinboxes such as trailair or mor/ryde.
That being said, I do not know what effect the gooseneck and adapter would have on the dynamics of trailer motion. It may be amplifing this effect. I would wait and see if anybody else on this forum has used a similar setup. I'm also sure that some of the more mechanically inclined members will jump in with a better explanation of rubberbanding.
Fred
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