Mike,

I think I know the feeling you are describing, but I'm not sure I know the cause. I experienced something similar while driving my 850 from NJ to MO during a New-Years eve blizzard in 1998. The road conditions were awful, with hard-packed snow in the lanes, and large ridges of loose snow (6+ inches high) between the lanes and between the tire tracks.

The car felt like it was drifting / sliding from side to side at any speed over 10mph, without the steering wheel moving. It was very disconcerting, as I recall, since the net effect was that the steering response seemed slow, and unable to correct the side-to-side motion.

At the time, I attributed the problem to the tires I had on the car. They were Michellin Pilot XGT Z4's. An excellent performance tire, but very bad winter tire.

I now have Michellin Pilot Alpine snowtires on the car, and have not experienced the same feeling yet this winter. (I love these tires in the snow!)

The only other thing which could have had some effect was, as I recall, the fact that the entire spring / shock absorber assembly had become jammed with snow, which in turn had frozen, so the car had no shock absorbing capacity at all. Actually damaged the engine mount from the pounding.