Not true on the tongue weight, mine has less than 100 pounds and can be towed at 100 mph. It is where it tips that matters, you can hold mine with two fingers at 23-25 degrees.. Designing the trailer to work around the hard items that do not change is the key, keeping water and liquids at axle heigth makes a lot of difference. The key to towing a trailer at high speed with correct tongue weight is to lower the front of the trailer 2 inches below the rear, it changes the trailer completely.
Still think this was more to do with something seizing up than trail conditions or suspension causing it. And you should lower the trailer tire preasure to remove the bounce. I very seldom run mine above 25 pounds and go down to 12 to 15 on serious trails.