I currently have the same problem and having spoken to a number of mechanics, including some BMW "technicians" as they like to be called, there are two common problems.
1. The thermostat gets stuck and pressurises the cooling system, this can be quite apparent if you turn the heating to hot and it doesn't get hot even when the temp gauge is on red. Another sympton here is when you remove the cap from the reservoir the water bubbles out like a geezer, it's because of the back pressure but most less knowledgeable mechanics like to tell you it's a sign your head gasket has blown or your head is warped or cracked !!!
2. The fan in the water pump. They had a probelm with the plastic used for the fan so it has been revised twice, once into a metal version and again into a stronger plastic version. This failure normally shows itself if the car has been gunned down the motorway and the water pump has had to try and pump the water around more quickly. the older type of plastic fan would strip on the spindle allowing the spindle to spin but not the fan at high speed. Hope this helps.