Why does my steering wheel move without wheels turning while driving?

When I start driving the steering wheel will be in the normal position, then the steering wheel will loosen change positions and tighten back up, but be turned nearly completely upside down. The tires never change position when it happens, but the vehicle will jerk to the right sometimes when this happens. The change occurs when I accelerate, turn, or slam on the brakes. The steering wheel will then correct itself if I tap the brakes or hit a bump in the road. I replaced the entire front end of the car and it did not fix the problem, although it did seem to help a little.