I have had a problem with my 55 reg plate 1.8 TdCI LX focus, recently I have noticed that I suffer dips in power while driving but had put it down to the possibility of me taking my foot off the accelerator slightly. Last night however I was driving home and as I was approaching a roundabout at 40mph in 5th gear I changed down to 3rd gear then to 2nd gear and the engine systems fault warning light came on. The car then started "juddering" and would not go above 20mph or 2000 revs. I pulled over after crawling round the roundabout and turned the engine off. I sat there for a couple of minutes and when I turned the engine back on the warning light did not reappear and the car was driving as normal.
The description of the problem in the hand book is "A malfunction of the engine and powertrain related systems"
Any advice on what this could be would be most appreciated as I would like some advice before taking it to a garage.