I have an opinion or two...
I believe it is definitely related to the worm gear in the electronic assist unit.
What I don't understand is why the symptoms don't occur when the vehicle is new. From what I have read it seems to happen to cars several years old. Perhaps someone can tell me I'm wrong and their car suffered when new? If it's true that new cars are OK then what changes in the worm gear with time? Would dismantling, cleaning and new lube fix it for a few more years? Is this possible?
It gets worse at high speed as the steering boost is diminished (variable boost)
It definitely gets worse if the car is parked in the sun with the roof and windows closed but why would new cars not have this same heat issue? It seems to happen after driving for a couple of hours even in cool temperatures so time seems to matter, perhaps time creates heat in the servo and worm gear.
There should be a general recall for this issue as it is a design fault...the NHTSA let the owners down big time...perhaps they were afraid of being litigated to death by BMW.
Shame on BMW for not stepping up.