I have a 1989 420 SEL ,vibration that comes and goes, had all tires balanced no bent rims, no bad tires,new shocks,new front struts,tie rod assy. rebuilt steering box, vibration is not speed or RPM sensitive (really noticable at or above 70 MPH), seems to be thru the center of the car, feel in steering and seat at same time, thats why I say center related. Thought it may be AC clutch related, but turned off and on did not make any diff. Called the dealer cann't seem to help. Checled u-joints,flex plate rear end pinion bearing for play found everything OK. ANY SUGGESTIONS????
All help would be very much appreciated.
Thanks for your help
PS 128K miles.