The dealer wanted to replace the whole steering rack, but couldn't guarantee that would fix it. They checked and believed there was no issue with the steering column, but the more old threads I read, the less certain I am.
Looks like there are many previous threads that may or may not be the same thing.
http://forums.pelicanparts.com/porsche-911-technical-forum/359332-steering-column-locks-while-driving-help.html
Notice this is the same person as above, 5 years later, so I can't tell if it's the same problem finally solved or a different one:
http://forums.pelicanparts.com/porsche-911-technical-forum/689853-steering-wheel-stuck-while-driving.html
"Sounds like you may have chipped a tooth in the rack.
There would be a small piece of tooth floating around in the grease as well."
"The problem was with the universal joints in the smugglers box. I just packed some grease in the joints and the problem went away."
"The steering rack was being interferred with by an alarm horm which was not mounted and was loose under the front suspension pan. They reworked and secured the horn."
http://forums.pelicanparts.com/porsche-911-technical-forum/343319-steering-wheel-lock-up.html
"Turned (no pun intended) out to be a bearing feel out and was causing it to stick."
"It turned out to be a bearing that was completely worn inside of the steering column. I do not believe it was thhe one that is immediately under the steering wheel...it is deep inside the column/shaft."
This problem description seems the most consistent with mine:
"Occasionally, on hard right hand/left hand turns, my steering wheel locks into place at completion of the turn! This is quite scary - the only thing I can do is hit the brakes (looking like a nitwit turned into the curb, and hoping there isn't a hydrant, pole or pedestrian standing there!) - once the car comes to a complete stop, the steering wheel unfreezes..."