P38 diesel running problem

I have just bought my first p38 and i love it apart from a few initial problems. The main one im having is that the car seems to be running a little ruff when crusing it seems juddery/hesitant or kangarooing. The injector light flickers on occaisionally on overrun when changing gear or slowing down on engine breaking. Then this evening on the way home from work it almost died as i was overtaking. i was changing gear from 4th to 5th when it suddenly lost power and would not pick up infact the revs dropped right down despite my foot being planted on the floor. The injector light was on fully at this point not flickering. I then changed it back down to 4th gear and the problem and light went away. it was back to normal for the rest of the journey just the juddering etc. Any ideas what the problem is? it was plugged into a comp at my local specialist at the weekend and no faults came up???