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After 15 years, I would be looking at replacing the most unglamorous part of the suspension - the bushings. I rebuilt a '93 BMW 325is after about that long and when I replaced the bushings, it was truly like a new car. The problem is that bushings are cheap, but labor intensive. Stock rubber ones are more comfortable, but low-hysteresis polyurethane reduces the time between input and output like no other change you can make.