[X(] Advice needed badly

I own an a4 1.9Tdi and have done for some 9 years now with 164000 odd miles on the clock (of which I did approx' 90 of them) it has been an excellent car. However over the course of the last year I have noticed that the drive has gotten progressively worse. When crusing the car spits smoke and kind of kangaroos(jerks) a little. The car also has a flat spot at about 80 mph.

I have changed all the filters, to no avail - also it recently passed the emitions test during MOT.

Has anyone out there any clues as to what the proble could be i.e. a fuel pump or something else?????