My old Classic developed the waggles on one test run. I looked at the hitch and saw that the centreline of the drawbar was below the towball height and assumed that was the cause (360 degreee swivel hitch). I tried simply bolting a bit of plate between the hitch and drawbar to take the 'swivel' out of the hitch and never saw the problem after that. But then I didn't take it far after that.