There are a number of things that affect ride. Start with tires. Rule of thumb is no more than 10 yrs old. Pressure: set it accurately at 30 psi. Ck your spare too.
Shocks/
struts: a fully assembled strut has the new shock and spring in it. You have a choice of gas or oil. I suspect the gas
shocks would give softer ride but have no experience here. Suspension
control arms: there are 8 each with a combination of
ball joints and rubber
bushings. These can be inspected but 20 years rots rubber. Seats: these deteriorate and the internal structure can come apart, the foam deteriorate and even the seat covers of leather can pull apart especially if the previous owner was heavy or used the car as a daily driver. Frame cross members: there two "X" members under the car that are bolted on (I have read there are 3 but I could only find 2). These bolts should be torqed to 46 ft lbs (I have read). Some of mine were loose.
Sway bar tie rods: There are 4, 2 front and 2 in back. They each have a
ball joint at each end. They connect the
lower control arm to the
sway bar end. The boots will be rotted and the ball jts. could be loose.
Sway bar hold down brackets: these hold the
sway bars to the frame. I have not looked at these yet. Steering ball jt.
tie rod: one each side. Boots will be rotted and ball jts. could be worn. Alignment
tie rods: there are two at the back wheels only. There is a ball jt. at each end. The boots will be rotted. They run parrallel to the rear axel and connect to the wheel hub and the frame. They create a parrallelogram with the axel and are adjustable to align the rear wheels. This is why this car needs a 4 wheel alignment. Shock towers: Make sure the nuts holding the top of the shock/strut are tight. You have to remove all of the trunk liners to access the back ones. That's all I can think of and have worked on all and presently redoing all 8
control arms.
All this can be inspected.
Ball joints must be removed to be checked for loosness. Rubber boots can be replaced on the bench.
It all has to work together and seems to me to be a high end suspension system used also on the Lincoln S series an Jaguar Type S. That's a pretty good pedigree and I am trying to get back to it. It can be $3 or 4K for suspension work and up to $3K for seats if recovered. Good luck.
