I would check the straps that holds your car . Its normal to have the straps loosen a little after a few miles , but not to that extent . Maybe there's a tear somewhere in the strap , OR your not tightening the car down enough . I look at my tires and make sure they bulge aliitle when tightening . I always put more air in the front tires before ( 40 lbs ) to stop alot of flexing in the tire .
I hope this will help you solve your problems .
Happy travels , Tommy Tucer .