Why does my engine light keep coming on??

Ok here goes, Original problem was everytime I overtook another vechicle i would drop a gear get power from engine and overtake. When overtaking car would loose power and engine light came on. Car has been in garage (fixed by a citroen trained mechanic) put on auto diagnosis machine saying diesel sensor was faulty. Had new one fitted and still had same problem. Took car back to gargae they tightened the screws in the sensor which is now ok. When I overtake I now have the power but the engine light keep coming on. Car has been in garage 5 times for the same problem but they say there is no fault/they cant find the problem even though they said they cleared the fault when the new sensor was fitted. I cant understand why I am having this problem all the time. I could just be driving down the motorway say doing 50 - 60 mph and the engine light comes on - WHY???? I have also got a tinny/metal ratteling noise from under car just had new exhust fitted but they say it is fine - Again WHY am i getting this noise. I tend to hear it more when I am going over a bump or pothole. PLEASE HELP!!!!!!!! I have a citroen xsara Lx 2 litre diesel year 2002.