As was said above, bounce and porpoising are two different things.
You said you have bounce so lets look at that.
Dolly's bounce when there is no load on them because they have no suspension and the tires act as such.
Small trailers do have suspension but they do the same thing as a tow dolly when there is no load in them because of high spring rates.
Two things you can do.
#1. Lower the air pressure on you tires so it will cushion the ride and act as a suspension component.
#2. Get some shocks on the trailer so the shocks will control the rebound. You will have to pick the correct ones. Ones with too much compression and you will be right back to bouncing all over the place again.
Good luck and happy travels.
