OK - My guys (BMW of Austin) have come up with the same solution as yours Relinius, (the auxiliary fan relay). They say they definitively have solved it. Although randomly, they couldn't get it to reproduce this time, once they started going down the relay path, they unplugged the aux fan completely and then could get the problem to reproduce instantly. Then they were able to replace the relay, and, according to them, problem fixed. They did not mention faulty wiring diagrams, but I didn't ask either. As they described the problem to me in laymen's terms, they said becasue the relay was intermittently faulty, when it was not working the car would cut back on power as a engine safety measure (ie, to keep it from overheating). In other words, the car is smarter enough to not overheat itself if the fan isn't working. The diagnostic problem was that because of the intermittant nature of the relay, no codes were being throw. Sounds logical to me (I think).
Anyway, I'm on my way to pick up my car. I'll see what happens, but I'm assuming it is fixed.
BTW, I've ran over a nail and my guys replaced the runflat tire immediately, no questions asked. My service has been great so far. BMW of Austin (owned by Penske) is really customer focused.