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