Why is my 1.6corsa sport cutting out?

I ave a corsa sport and wen i bort it was told that may need new egr valve well wen i start the car its fine, when im driving its fine but when u go to slow down at a roundbout for instance it cuts out n i hae to start it again sat there but it starts strauight away. Then it were idoling funny so fitted a new idle control valve as well as an egr valve but the problem still persists! does ne one know why it wud be cutting out on me also speedo does work sumtimes n half way on a journey it randomly stops working! n the ecu light comes on not all the time but now and again to does ne1 have a clue like as to what wud be causing the car to cut out n what should i do as i have a mot coming up next month n no way is it gunaa pass at all :( ????