To the OP I am experiencing the same thing here in Detroit. I have been chasing steering suspension issues since I got here (originally from Kansas.) I have replaced my control arm bushings with powerflex bushings and it helped however it still wanders like crazy. I will continue checking until I find a solution however I am beginning to think that tramlining is my issue.

Might I add, I am running non runflats.

 

There is nothing wrong with your car, it's the tires. As others have said your current tires are less than optimal on grooved roads.

My experience has been that with the stock factory run-flats, Dunlop SP Sport 9000 DSST, there were no issues with tramlining at all.
When I replaced the run-flats with a set of Dunlop Sport Maxx I started having tramline issues.
I've found that a couple of pounds of air pressure over normal, 37 instead of 35, in the front helped some.
Having said that I was still unhappy so I replaced those with Michelin Pilot Exalto PE 2's and have had less issues with tramlining.
I still have some tramlining but nothing like with the Sport Maxx and frankly not enough to worry about.

So try the air pressure and if that doesn't work think replacement tires.

Hope this helps...