Being new to this forum I have only just seen this. Before spending any money on cooling systems, always try changing the pressure cap first. If its not holding the pressure it will give you the same symptoms you are experiencing.
Just wanted to say hi. I'm Bruce from Essex. I have owned around 50 cars over the years but this is my First Beamer. Just bought a low mileage 2008 320i M Sport Coupe on the weekend after selling my 11 plate Focus Sport which I had owned from new. Look forward to learning more about BMW's and hopefully will be able to contribute from my experience with cars over my 35 years of driving.