2004 Grand Marquis Very rough riding in cold.
 

My GM has a very rough ride when starting out on a COLD morning. This roughness is severe. A bucking motion in the rear. Almost unsafe at 40 or 50MPH. After 10 minutes, it smooths out. I've had the air suspension checked out and considerable repairs done to replace airbags and the compressor. Still, the problem persists. The shocks seem ok, but they are older. I replaced the sway bar in this car with a 1" model years ago, and it didn't cause the problem, but it's the only change I've made to the suspension.
Suggestions?