We had a guy at our field that had a UCD that would porpoise and I noticed that whilew he was running up his motor the elevators twitched. It turned out that the extension between the elevator servo and the receiver had a bad connector. When the engine was throttled up and down the vibration would cause the plane to bob up and down erratically. He probably flew it a dozen times and was about to throw in the towel when we noticed it. Replaced the servo and the extension and the problem went away.