First, check to see that the tires are round. A quality tire servce shop, *not* Winston, can check to see if the tires are true and true them if needed.
Second, have someone knowledgeable and capable check your wheels to make certain that they are round and true. It only takes a minor 'curb surfing' episode to throw things out of whack. If your wheels are 'square', they will never balance properly.
Third, have the wheel/tire assemblies balanced while mounted on the car. Do you use locking lug nuts? It doesn't take something that large to throw off the balance and balancing on the car can often compensate for problems that do not show up otherwise.
Finally, have a proper four wheel alignment performed at a MB dealer. It is worth the few extra dollars to have the job done right the first time.
'76 Moto Guzzi 850 Le Mans