I say not enough tongue weight. Going rounds now with a guy that bought a new trailer from me recently, trying to blame the trailer for whipping all over the road AND GET THIS, taking out his driveshaft also, What an idiot!! He tows with a 73 Blazer thats been lifted then has his show truck,a 68 or so GMC truck loaded right at the back edge of the trailer. No W/D or anti sway control either!! THEN, where he has this thing tied down waaaayyy under ,damn near in the middle of the truck for christs sake from both ends!! I offered to go down to the local Highway patrol and see if they would tell him whats what, but he won't so basically I told him to go get some opinions from people and see what they think. If he calls again, i might take a picture and post here and let you guys tell him whats wrong with his set up.