Hi, I have a 1997 1.9td Ducato also with the same problem, I discovered all the fins on my radiator were damaged, causing my vehicle to over heat. I fitted a new radiator and problem solved. You have to watch out I sourced a radiator on the web, it fitted but was a lot bigger than the original so I had to discard the plastic plate between radiator and inter cooler but I've had no problems since and the vehicle runs better as well.
P Bowden
August 2009