warning light come on and cut of my corsa?

Hi i have a corsa classic 1.6i 2001 model , my engine warning come on at first the garage changed the crank censor as they said that was at fault but the problem is still there nw, the light comes on and cuts off the car after a few minutes when i start up there is no idle, so i switch of start again and idle comes back to normal, then at time the rev flucuates, im so fad up now cos there seem to one problem after ther other and everytime i have it cheked out its costing more and more money, please help