I have a citreon c5 diesel that's done 210k. Intermittently when I put my foot on the accelerator it feels that there is a blockage which restricts my acceleration and then it clears with a thick plume of black smoke coming out the back and I accelerate as normal. My regular garage found a couple of split pipes which they have replaced but it has not resolved it. They were unable to give a clear indication of what it might be or how much it might cost to put right. Any suggestions what the problem might be?