God, thought I had written thisa post myself. I have identical problem. Are you sure I don't have your car and the same mechanics.
I've just changed the main fan belt and tensioner, fuel filter, air filter and full exhaust including cat. It's still the same. The only difference with my car is the engine warning light comes on now and again when going up an extended incline. It hates hills.
A diesel mechanic did say the same to me - fuel pump (big bucks), may be an injector but check loom first as these are a problem. I have girlfriend car at moment so having spend 2 times more than the car is worth it is sitting there till I need to give her her's back.
Do you know exactly what it was eventually, nodoubt mine is the same and it may save me time and money to look at what ever was wrong with yours first.
Thanks