just recieved an iphone from a frnd .. dunt knw which firmware but has a strange problem which i haven't seen yet ... start the iphone .. apple logo for hardly 1 sec.. then screen goes black .. and after that keeps flashing white/black after every 2 sec ... kinda like a tubelight trying to light up .. tried DFU mode .. but pc says USB device not recognized.. been searching..but didnt findanything .. any ideas ?.

and i tried changing the ccable and dock but no luck .. the cable works perfectly on my ipod ...