Usually it is because the frame the body is mounted to, flexes and bends. Only sure fire way to prevent it, is a properly reinforced frame. Other steps you can take to minimize the flex of the body is to apply lots of fiberglass resin to the inside of the quarters to stiffen them. I wouldnt reccomend solid mounts on a daily driven car due to noise amd vibration harshness. Replacing the rubber mounts with urethane ones though, wouldnt be a bad idea.
Take it from one who knows, am in the midst of fixing my frame and straightening out my quarters on my caddy now :biggrin: