I have a 2000 323i E46..this problem just started occuring. I had not run the car for over a month because of snow and cold here. But, I got it out a cople of days ago. I noticed that the temperature needle was not coming out of the blue after about 5 miles of driving....then all of a sudden it went right into the red...I pulled over, shut down the car and checked for overheating, but everything seemed normal. When I restarted the needle went back to the top edge of blue and stayed there the rest of the trip (another 15 miles), I topped up the coolant about a pint when I got home. It did the same thing today...but after I shut it down and restarted it was back to normal. Would this be a faulty temp sending unit or faulty thermostat? Or what else?