Porpoising when towing on Concrete Roads

We are new trailmanor owners and have a 3124 that we tow with a 2001 Chevy Silverado pick up. On our first outing, we noticed a problem towing on concrete roads. We had a good smooth ride on asphalt, but when we hit concrete roads we started porpoising. We tried speeding up, slowing down and nothing helped. We have a weight distribution hitch and we were towing with empty water tanks. We were not carrying a lot of weight in the trailer. We are wondering if anyone else has experienced this problem and how they resolved it?
Thanks,
Carl&Judy