I second the weighing of dry ingredents... FAR BETTER... Should have done it my whole life.
I think the problem with the cookies is the margarine. Not all margarine is created equal, especially in the UK, and most of it in the UK isn't suitable for cooking in cakes and pastries, because what they do is put large amounts of water in to reduce the fat content and make it "more healty" but what it will do is ruin your cookies.
I had to explain this to my mother, who had been cooking for 45 years, ran out of stick margarine (which you can't even get in the UK!) and decided to use her tub margarine. She had made some sort of cookie or dessert and it had come out flat and hard... Sound familiar. What type of margarine did you use?
oops... Re-read and realised you said you were using butter now. I wonder if your recipie accounted for the higher water content of margarine that that is why the butter isn't working... *shrug*
My suggestion is look for a margarine that is labelled "suitable for cakes" or "suitable for shallow frying" or just go for the butter, because for Christ's sake, they are cookies, right!?