I have a 2003 BMW E39 525dSE auto Touring that appears to have a problem of overcooling. It can take up to 10 miles to reach normal working temperature, summer or winter. At that point, descending hills or time spent idling will result in a fall in the temperature gauge reading. I have had the thermostat checked by my BMW dealer and, although it was working correctly, I had it replaced in case it was sticking in a partially open position. This has made no difference. I am wondering whether the fan is cutting in too early. It might be a problem that you have come across with this model, and I would be most grateful for your thoughts on this matter.