Was the Autocross track really bumpy? Maybe body roll and bumps cause the tires to contact the body?
In Reno's Hot August Nights "drift car" course last year, there was a dip in the course to provide drainage for the parking lot. You'd see the cars sliding sideways through the curve and the dip would sometimes upset the car to the point of gaining traction or spinning out. The better drivers were able to fight through it and keep their line. I understand that many Autocross events are held in empty parking lots so you take what you can get.
How did you do otherwise?