Intermittent P0015 code

[mouncer]

2008 E70 3.0 with 84K Miles. I get a CEL every now and then for P0015. Usually goes away and comes back every month or so. No performance issues with the vehicle, runs like a top. Did the water pump a month ago and put new plugs and coil packs at the same time. So does not seem like any recent repairs caused the P0015, it was doing it intermittently even before I did those repairs. Searching on the forums would suggest it has something to do with exhaust cam shaft sensor or Vanos. What should I be looking for? Just ordered a BMW scan tool to perhaps give me a more specific diagnosis, but if I could get an early start on it before it leaves me stranded on the side of the road I would welcome any suggestions.