are any of the lights cracked with moisture inside which can affect the contacts?
are any of the bulbs nonOEM?
I had this problem, and sometimes it pops up and dissapears again... no fix yet. Just kinda ignoring it for now... but answering YES to above questions I heard would mean that the problem wont go away till those are adressed...
Stock '07 Z4 3.0si - Titanium Silver/Black/18s OEM 135M