Your tires may be out of round which theres really nothing you can do about it other then get new tires. An alignment may help but i aligned mine and it made no differance when the exact same thing was happening to me. And if it was a balencing issue bumps would not effect it and it would only be a slight vibe at high speeds. Before you do anything i would switch the front and rear tires and see if that makes a differance. I fixed my by doing that. Let me know i am very interested to find out wat it is.
Two questions, do you have a steering stableizer on it and does it go away if you go faster. It sounds kinda sketchy but it will narrow down the problem.
Hope this helps,
Anthony
