Flat spotting, happens from time to time especially with certain tire models. If you don't believe, then put your OE's on.

 

Had this on my previous new E60 M Sport with 172 19" wheels and conti tyres. you could see the passanger seat vibrating, the vibration was always worse from cold but never really went away. Had the wheels balanced and it made no difference. My current E61, again a new car, has 17" Goodyear run flats on and is the same but not as bad. Depending on where and how long the car is parked i get a slight wheel wobble / uncertain feeling in the steering for the first 5 / 10 miles, usually starts at around 60mph through to about 80mph. Infact nearly every new car I have bought has suffered from this problem!! I always fit Michelin tyres when the original ones need replacing and these seem to be a lot better.

 

I guess we found the solution for OP, just a matter of whether he will accept it or not