Over the years I've had two friends that have battled front end vibration problems on their W126 cars - for some reason W126's seem to be very sensitive to wheel balancing issues.

In my case both problems were ultimately solved by finding a shop that could balance the tires with the wheels on the car. Not very many shops have the equipment to do this but if you look around you might be able to find one in your area.

If you can't find a place that does wheel balancing "on the car" go back to the garage that balanced your tires last time and ask them to do the following check. 1) balance one of your wheels on their machine 2) rotate the wheel on the balancer 90 degrees and re-check for balance. The wheel should stay in balance after it has been rotated, if it does not it indicates that the procedure for mounting the wheel to the balancer is not providing a consistent mount. The best balancers use a "pin-plate" mounting jig that secures the wheel to the balancer just as it is mounted to the car. These systems are the best way to mount a wheel on the balancer and in my experience they provide consistent and repeatable results.

I like Brian's idea about checking for tire roundness but I would also remove the tires from the rims and check for run-out. If have seen more than a few bent rims over the years, especially with aftermarket cast wheels.