It would SEEM like your spring bars on your WD hitch are likely the culprit. With spring bars not stout enough to handle your tongue weight, porpoising is the most common effect. Spring bars should be sized for a MINIMUM of the actual weight of your fully loaded trailer. If you don't have weights for the loaded tongue weight (not the empty weight on the brochure) then use 15% of the GVWR of the trailer. When the size is right and the adjustment is right you should experience ZERO porpoising without air bags or helper springs.
Good luck / Skip