when the engine management light comes on?

i have a 1999 1.7 turbo desiel astra and the engine management light has been coming on for the last TWO years! when it's lit, the engine goes into idle mode and loses power, so to get rid of it i drop the revs or change down a gear to up the revs between 2000 - 2500 and press down on the accelerator and the light goes out and the engine picks up and runs fine until the next time. i cleaned the valve out it had oil in it?? i don't know why!! my main question is, could a senser be faulty causing this problem?? before i go spend spend £800 on a new one. many thanks if you could take the time to help!!