Try adjusting the tire pressures to ge tfront/rear slip angles matched. First raise rear pressure by 2 psi, then another 2 psi to see if it improves. You sould be runing on the high side of recommendations in any case. BE SURE YOUR TIRE GUAGEis accurate. Higher tire pressures do wonders for handling

Then check rear wheel bearings for any play.

Have you added or removed any body parts like a spoiler or sun roof wind deflector. One thing for certain this wandering is not inherent in these cars.