Was pulling a 10 ft. landscape trailer (flat non enclosed trailer) with two motorcycles on it. When we got to about 55-60 mph the trailer started to go very unstable (side to side motion). It felt like it was going to pull the car right off the road. We've used this same trailer before with no problems. Only difference was we both got heavier motorcycles (went from total weight of about 1200 lb. to about 1500 lb.). We moved one of them back a foot to try to lessen the tongue weight from the heavier motorcycles a little bit. Somebody told me that instability was caused by too little tongue weight. Does anybody have experience with this? What causes the trailer to go unstable and how do we fix that. It's not fun driving 50 mph on a 1000 mile trip. Air pressure in the tires is fine.