Shortly after purchasing a 67 F Model I experienced severe nose wheel shimmy on takeoff. Removed wheel and inspected tire, bearings and reinstalled. Next flight shimmy came back on landing roll out with no braking. Removed wheel, rebalanced, same result. Installed new tire, same. Prior to this we did 6 take off/landings with no shimmy and no prior history. Shimmy has been only on landing since first incident. Would appreciate any input to helping solve the issue.