I have had a lot of experience in running and maintaining deisel engines in cars and vans although I'm not a mechanic. The problems you describe sound very much like what happened to my Renault Traffic a few years ago when one of the cylinder wet liners broke and allowed coolant to boil off through the exhaust, resulting in a black sticky deposit on the rear window. The problem seemed to resolve itself for a short while but ultimately resulted in a new engine being fitted. I really hope you can sort your problem out a bit cheaper than that. I think that any Ford garge could make a proper diagnosis.
Best of luck
Pete