Tires and/or shocks are the culprit. After you determine that you have the correct tire pressure and the shocks are not leaking or worn out, the best thing you can do is to replace the run flats with normal tires. If your car has the sport package, replacing the springs and shocks with the parts needed for cars without it should help too.
BTW, a new 2012 X5 with Adaptive Drive (an option) and normal non-runflat tires has the ride of a Rolls Royce with the handling of a BMW
