So met this friend yesterday and he showed me his lumia 520 which was lagging like sarkari babus. My brother has a lumia and it never lagged. He told me and showed me as well how when a call comes, the screen goes blank for 4–5 seconds, and when he try to repeatedly open and close default camera app, the visuals on screen resemble those on TV when the antenna moved from its rightful place.. Even after soft as well as hard resetting the phone, the problem continues.. So what could be the problem and its solution.. Also he has whatsapp and Nokia apps installed.. So i don't think this is coz of any of them..