Vauxhall Corsa power cutting out

Hi, please help. Went for a run in my corsa the other night and when I put the foot down the engine power died sharply at just over 2000 rpm and a light of a car with a spanner through it came on. When I lifted off the light went out. This pattern repeated itself many times! The car had only done about 70 COLD MILES in the last month. Reading the owners manual I realised it might be the fuel filter so I RENEWED it. However the problem was still the same. Went for a 20 mile drive and when the engine was at max running temp trashed the arse of it! The car was slow getting up to speed but got there in the end. When I got home the light(car with spanner) was on all the time. Stoped the engine and restarted it, this time the engine management light came on and stayed on. Took the car for another run and it was back to normal, however the EM light is on all the time now. If someone out there has also had this prob or know's what has caused it please let me know. I am a time served plant fitter and know my way round a diesel engine, however I need to know where to start!!! Thanx.