If you have to stop, and then it goes away, probably until you hit another jolt, then you've got a king pin problem. Could be the pin itself, or the bushings in the spindles (4 total), also there should be a trust bearing between the axle and the bottom flange of the spindle on each side, with some thin washers to take up slack. All that should be inspected. A damper is only for minor "undulations", your's sounds more severe than that.