I had a similar problem last year. I spent a lot of money on diagnostics and a lot of time trying to figure it out. In the end, it was a blocked pisser hose, remedied by pushing a paperclip into the little hole in the expansion tank and wiggling it around. Problem solved. The obstruction was around #9 in this diagram:
It may not help you, but I'd start there, since it's a very easy and very free thing to do.
Here's the thread I posted that eventually lead to a solution:
http://www.bimmer.info/forum/showthread.php?t=12836