Vibration on CA highways 1994 S500

[gurban]

Here's another strange one for me. On certain highways here in California I get harmonic vibration in the suspension. Brakes don't stop it. I have to slow to about 35 and then can accelerate back to freeway speeds. Sometimes the vibration comes back some times not. Tires and wheels all balanced. Shocks seem to be OK, pass the bounce test. Car has 151,000.
Ball joints? Bushings? Hydraulic suspension? What should I check first?

Thanks,

Gary

[dima4855]

I would check the tires, just because they are balanced dosen't mean the belt didn't split.
Check your suspension by putting the car on ramps, check for obvious loose/broken parts.
Describe the vibration a little better and where it is noticed the most in the car.

[gurban]

The vibration almost seems like a shock is out. Front end. Bumps instead of wiggles if that makes any sense at all. The highways out here tend to be cut with groves. Makes no sense either since there is almost no rain and certainly no ice. Living mostly now in Southern California outside of LA.