530D vibrating

Hi, I have got a BMW 530D touring, an european car from mai 2000. Between 90 KMH and 120 KMH the car is very "hard on the road" and I can feel
every little bump on the road. This is very tiring and not how a BMW should ride. We have balanced the wheels already 2 times and also checked the stand off the wheels etc. etc. I know that the car has standard air shockabsorbers.
Can somebody tell me what I should do to get a more "easy" ride.
greetings Jan
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