I have just purchased a 1957 olds super 88 hardtop , the car has a vibration above 50 mph ,have put new tires & wheels on had them rechecked for balance are ok - had drive shaft balanced it still has viration over 50, just fine under 50 mph, drove car to 50 mph put car in neutral mantained engine rpm viration went away,put back in gear viration came back with in 10 feet of travel. Have checked moter mounts & trans mounts. Any help would be appreciated. THANKS SID