my mustang veers to the right and left on its own

My dealership can't replicate the problem. Even drove with me, witnessed the problem then said there is nothing they can find. I feel unsafe in the vehicle. also has a clunk sound when I steer a tight turn as in backing out of a parking space, both sides. Still they can't get it to do it so there's no problem.

Update from Dec 3, 2015: So I took the car to the dealership. A technician road with me and experienced the pulling. And yet they found no problem. They provided an alignment but could not get the vehicle to pull or hear the clunk which is intermittent. It continues to pull to both or toons and almost feels as though the steering "takes control" for a few seconds sending the car off to the right or left.