vauxhall zafir diesel 2004?

on the dash board a car with spanner is showing up vauxhall diagnosed the egr valve and glo plugs but wanted a fortune to change so i got my local garage to change the light still showed up so he said it might be hte air flow meter which we also changed and the light is still showing up but now the problem is the car doesn't have power in the middle of driving the car it drives fine and then the power slows down does anyone have any ideas or had the same problem??