Short of a complete suspension rebuild it's tough to point out any single cause, however, worn tires loose their ability to damp out high frequency road input and the car slowly becomes more harsh as the tires wear and harden. Bushings also slowly harden and degrade, but a complete suspension rebuilt is a very expensive proposition.

If your tires are well worn a new set will often noticeably improve ride quality by reducing harshness. Since the original fitment was 65 series touring tires, lower profile high performance tires will almost always cause increased ride harshness.

The deCarbon type shocks usually maintain their damping characteristics until end of life as evidenced by oil leaking.

Duke