i experience the same problems, i have always chalked it up to being the size and weight of my rims. i have 20 inch rims and they weigh around 40 lbs a piece. im sure the tires are a big part of the problem too. when i have my winter set on i dont get any issues (245/50/17). but my steering wheel has a little play to it as well, about 1/4 inch each way that makes the car feel a little wobbly. not sure what causing that though.