I have a blog which was doing good but now it's going down pretty bad after penguin 2. I used to get around 5k unique visits which has now come down to 1k and still reducing . My main concern is about copied articles. In starting I used to copy all good articles related to my niche which was working good for me for around 6 months but now they are no where in google. I think this is causing the downfall of my blog. now my question is whether to delete all copied post (around 150 out of 800 posts) or should I go changing each one by one which will take 3 or 4 months?