Fishtailing or whipping is not normal. It is caused by mis-loaded cargo. Speed will make it worse. Slow down, get to a safe place where you can reposition your load and put more weight in the front of the trailer. A good rule of thumb is to put 60% of the cargo weight in the front of the trailer. Use plenty of tie-downs to keep the cargo from shifting while you travel. Watch this short video to see the impact of mis-loaded cargo. http://www.kaltura.com/tiny/9oep0