We bought our 2007 G35X new, have loved it, and now have 85K miles, but it still looks like new. Ride quality has gradually deteriorated to the point that every road seam or bump results in a bone-jarring thump and clunk. It's as if the suspension bottoms out or there are no bushings. All four corners are affected, so I replaced the front shocks with no improvement, and therefore didn't bother with the rears. I paid for a tech at an Infinity dealer to inspect the suspension system and he said bushings and the fluid damping system were fine. Then I sprung to replace the (age-hardened?) tires with the OEM's that came with the car: no improvement.
I probably should have posted this before all the steps I've gone through, but would appreciate any advice or ideas that would help us avoid selling or trading this beautiful car.