2004 Mazda Premacy black smoke, lack of a acceleration at 2000rpm

My car is generally sick and Mazda have no idea how to fix it. Initailly I have been having black smoke emitted when I press the accelerator hard. The car starts fine but has no oomph. When the revs are approaching 2000rpm the car starts to judder and I have to force the car which either works followed by black smoke or it doesn't and the car increases speed at a slow but steady rate. Mazda have changed the MAT sensor which had no effect. I have changed the cam belt. Mazda sent the fuel injectors off to a specialist who said they were weak but that doesn't explain the lack of power. My car is also eating diesel like there's no tomorrow. I got 270 miles out of my last full tank. Mazda said the EGV(?)valve was working fine. The engine warning light comes on intermittently but goes off on its own and there is no code left in the system when I have diagnostics done. The air and fuel filters were last changed in April. Does anyone have any ideas? Thanks