I spent thousands chasing the same problem. I rebuilt my steering rack, new tie rods, new torsion bars, corner balance all new rubber suspension components, new trailing arm bushings. The list goes on.. but you get my point. In the end it was bad tires. My tires Michelin Pilot Exalto with 3k mikes on them were the culprit. Tire rack along with Michelin was amazing and replaced my tires under warranty.
My Porsche drives better than new now
)))
Good luck! Hope this helps you out.