I had talked to you about this on the e60 forums i believe. I had the same problem. went back and forth with the dealer. 04 545.
Ended up being the valve body (BMW calls it mechatronics). They said there is a service bulletin on it.
Total repairs were around 5k, had to fight like hell, but third party warranty eventually covered it. BMW originally said they would pay for part and discount the labor, but then said they changed their minds becuase I was not the original owner.
Car drives great now.