Engine Management light coming on?

Hi. Can anyone help me?
I have a vauxhall Astra 1.7TD. The engine management light is coming on. When you start it the light goes off, after a while the light comes on when you rev it or accelate, it goes off then gradually it will come on again.I did replace the Mass Air Flow Sensor but it made no difference. The car dosnt lose much power but you can tell. If anyone could please assist.
Many Thanks