I have a 2001 Type S, this spring I put on new 235/50 Exclaim UHP tires. When the car isn't fully warmed up and I get on the highway the steering shakes once I hit 100 up to 120 kilometers/hr as if the wheels are really out of balance, after driving for 10 or 15 minutes all the steering wheel shaking stops and the car feels fine. Anyone have an ideas as to what might be causing this?

 

The tires are developing flat spots as the car sits. Once the tires warm up the tires become round and the vibration stops.