there needs to be weight on the tow ball at all times , if the trailer is pitching it is too light .EG have you removed the spare from the front draw bar or moved the weight on the trailer somehow , bought new tyres that are bigger and lifted the trailer just enough to bounce up and down on the ball. try lifting the suspension up on the car (if its on air and not springs) so its caring the weight on the tow ball and see if its any different . if you have put smaller diameter tyres on the car it would have the same effect. in other words its balanced out at tow ball height.

Also, try the trailer with a different towing vehicle to see if the same thing happens - same could apply if you can try another trailer on the P38 ...

Tried a 7'x4' unbraked trailer today, loaded with logs. Towed smooth as silk, wouldnt even know it was there.

Even over the really rough bit of road after the Queensferry crossing that had the whole car jiggling with the big trailer, there was only the slightest of vibrations. None of the horrible banging and clattering the big trailer does either.

Will be picking it up again later in the week unloaded, so be interesting to see how that compares.