I had a similar thing happen in my 03 V40 which may or may not be related. It was fine at high speeds but the minute I got stuck in traffic it would begin to over heat. The over heat condition could be somewhat managed by turning the cabin heat on high and having the passengers suffer instead of the car. Did some research and found out that it was the fan resistor assembly that was defective. It would allow the fan to turn on at low speed but was preventing it from kicking into high gear when the heat kept climbing. The unit could be purchased separately but the dealership needed some help finding the part in their DB.

In in the case of the V40 the fan resistor is located right on the back of the fan cowling. Looks like a long shotgun shell with wires coming out of it. Not sure if that helps you.