I'm guessing you cleaned the throttle body and cleared it? Maybe go through and make sure everything else is clean.
I don't think you could get me to drive a mustang in winter weather. Last year we had an inch of ice and I watched this one mustang get trapped at a gas station because he couldn't get up onto the road. I later saw it abandoned on the side of the road because he figured out he could get traction in the grass. My all-terrains didn't have any trouble though.
