This has been an issue on a large German F20 forum
http://www.1erforum.de/der-neue-1er-...ung-96115.html
Apearently, to save fuel the electronic servo cuts off when not needed, like when the steering wheel is held in the same position for a while (straight ahead or a long constant curve). When servo aid is needed you have to overcome an initial resistance.
The problem is supposed to be known to BMW.
When ordering my F20 luckily I had already read about this occasional problem, so I ordered the car with standard steering. Which turns out to be a real good one. No artificial feeling and very good communication between road and steering wheel.