I know it may sounds silly but how old is the engine coolant liquid ? Once in a turbo car I had trouble because of the old (probably way too much antifreeze) fluid. I'd replace with new, quality fluid and cleand the hoses also.
airflow isn't blocked to the radiator... radiator cap is the new design (there was a recall on the 8 series in 1996 because of the radiator cap problem) and no air in the system. Thats my basic test always.
Also check the radiator fan blades. Back in 1990 I had a 750i (Euro) and had similar problem and 2 of the blades were damaged and probably wasn't blowing/sucking enough air ... just trying to help ...