Quote Originally Posted by Guapo
I had the exact same symptoms.

http://www.bimmer.info/forum/showthread.php?t=16791

I decided to replace both temp sensors, the water pump, and the thermostat.

I don't know which part was causing it, but the problem is now gone. When all else fails, brute force method.
And of course the day I say this, it starts up again. Sigh. Guess I'll look at a coolant flush and fan clutch.