Symptoms are that is smokes a hell of a lot of black (that I can see) smoke and is completely gutless on any sort of gradient. (Overtaken by a 16 wheeler not long ago

Since I got it about a year ago, I've had the leaky diesel pump reconditioned and the injectors reconditioned aswell. That didn't make much difference

Is there anything obvious that I should be looking at here. Wondering if the exhaust has a part to play in this? Not being very mechanically minded, it would be great to hear some opinions of those in the know before I fork out any more moolaa...
Cheers. Sam.