How old is it? Rad's tend to be knackered by the time they are 5 years old. Mine has just had a rad put in, I assume its 2nd replacement rad as the car is ten years old.
Also, the speed of the radiator fan is controlled by two thermistors, if one is broken the rad fan speed will not increase as the temperature rises, it will be constant.
Have a look here, this might be of some interest to you:
http://www.alfaowner.com/Forum/tunin...-new-post.html